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What the coil is actually delivering — total, sensible and latent BTU from four temperatures and an airflow
A dry-bulb split across the coil tells you about half the story, and on a humid day it is the less interesting half. Air leaving an evaporator is doing two jobs at once: dropping the temperature, and wringing water out. Only one of those shows up on a thermometer. Put a wet bulb on the return and the supply as well and the whole picture appears — total capacity, how much of it is sensible, how much is latent, and the ratio between them. That ratio is the number that tells you whether the equipment matches the building it is in.
This works it the long way, from the ASHRAE psychrometric equations, with the barometric pressure corrected for your elevation. No standard-air shortcut, no assumed density, no chart reading. It also shows you what the familiar 1.08 and 4.5 constants would have told you, so you can see exactly how much the shortcut costs where you are standing.
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You cannot compute your way past a bad measurement
Six numbers go in and every figure above comes out of them. Airflow scales the whole answer, and the supply wet bulb sets the latent half of it — half a degree of error there moves the latent capacity by around a fifth. These are the instruments that produce those six numbers, and on this page they matter more than the arithmetic does.
Fluke 971 Temperature Humidity Meter
- Reads dry bulb and relative humidity, and computes wet bulb and dew point
- Covers both input modes on this calculator with one instrument
- Dual display means you can take the return and supply pair without re-reading
Fieldpiece ST4 Dual Temperature Meter
- Two probes, return and supply read simultaneously
- Removes the drift you get walking one probe between two locations
- Shows the difference directly, which is the sensible half of the answer
Fieldpiece STA2 In‑Duct Hot Wire Anemometer
- Telescoping hot wire for a proper in-duct traverse
- Airflow multiplies every BTU figure on this page, in the same direction
- A traverse beats a nameplate CFM by a wide margin
HoldPeak 866B Digital Thermo‑Anemometer
- Vane anemometer for register and grille face velocities
- Also reads air temperature, so one tool does two of the six inputs
- Useful cross-check when a duct traverse is not practical
Dwyer Series 475 Mark III Digital Manometer
- Measures total external static pressure to 0.01 in w.c.
- Gets you CFM off the manufacturer’s blower table when a traverse will not fit
- High static is itself a common reason delivered capacity comes up short
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Why a temperature split is only half the measurement
Air coming off a cooling coil has had two different things done to it. Its temperature has been lowered, and if the coil was below the entering air's dew point, water has been condensed out of it. A thermometer sees the first. It is completely blind to the second.
That matters because the second one is not a rounding error. On a humid day a residential coil can be spending a quarter of its total output on condensing water, and none of that shows up in a dry-bulb reading. Two systems with an identical 21 °F split can be delivering capacities that differ by thousands of BTU an hour, because one is running wet and the other is not.
Ed Janowiak, who runs HVAC design education for ACCA, puts the case for the enthalpy method plainly:
“If we know a pound of air has a difference in enthalpy supply and return numbers, which are obtained by taking a wet bulb temperature, with those wet bulb temperatures we can determine what our change in enthalpy is and with a little bit of math, we can determine how many BTUs a piece of equipment is removing… it's a great way to determine if a piece of equipment has the potential of doing what it's supposed to as opposed to simply taking a couple of sensible temperatures which I'm not in favor of.” — ACCA HVAC Blog
The extra measurement is one more temperature at each location. What you get back is the total capacity, the split, and a number you can compare against the water actually running out of the drain.
The equations, and where each one comes from
Everything on this page is out of one chapter: ASHRAE Handbook—Fundamentals, chapter 1, Psychrometrics. Nothing is fitted, nothing is a rule of thumb, and there is no coefficient anybody tuned.
Step 1 — barometric pressure from your elevation
p = 14.696 × (1 − 6.8754×10−6 × Z)5.2559 psia
Z = site elevation in feet · ASHRAE eq (3)
Z = site elevation in feet · ASHRAE eq (3)
This is the first thing the calculator does and the first thing most field calculations skip. Pressure sets density, density sets how many pounds of air a CFM represents, and the capacity equation works in pounds.
Step 2 — humidity ratio at each state
Humidity ratio is pounds of water per pound of dry air. It is the only moisture measure that stays put when you heat or cool air, which is why every step below is built on it rather than on relative humidity.
W = [ (1093 − 0.556 t*) × Ws* − 0.240 × (t − t*) ] ÷ ( 1093 + 0.444 t − t* )
t = dry bulb °F · t* = wet bulb °F · ASHRAE eq (33) and (35)
t = dry bulb °F · t* = wet bulb °F · ASHRAE eq (33) and (35)
Ws* = 0.621945 × pws(t*) ÷ ( p − pws(t*) ) ASHRAE eq (20)
0.621945 = 18.015268 ÷ 28.966, the molecular mass ratio of water to dry air
0.621945 = 18.015268 ÷ 28.966, the molecular mass ratio of water to dry air
If you entered relative humidity instead, the route is shorter: the vapour pressure is that fraction of the saturation pressure, and the same eq (20) turns it into a humidity ratio.
Step 3 — enthalpy and specific volume
h = 0.240 t + W × ( 1061 + 0.444 t ) BTU per lb of dry air · ASHRAE eq (30)
v = 0.370486 × ( t + 459.67 ) × ( 1 + 1.607858 W ) ÷ p ft³ per lb · ASHRAE eq (26)
v = 0.370486 × ( t + 459.67 ) × ( 1 + 1.607858 W ) ÷ p ft³ per lb · ASHRAE eq (26)
Those three constants in the enthalpy equation are not arbitrary: 0.240 is the specific heat of dry air, 0.444 the specific heat of water vapour, and 1061 the enthalpy of saturated water vapour extrapolated back to 0 °F. All three are inch-pound conversions of the SI figures ASHRAE prints alongside them.
Step 4 — mass flow and total capacity
ṁda = 60 × CFM ÷ v lb of dry air per hour
qtotal = ṁda × [ ( h1 − h2 ) − ( W1 − W2 ) × hw2 ] · ASHRAE eq (45)
qtotal = ṁda × [ ( h1 − h2 ) − ( W1 − W2 ) × hw2 ] · ASHRAE eq (45)
The second term is the condensate credit, and it is the part most field calculations drop. Water leaving the coil as liquid at the supply air temperature takes some enthalpy with it, and the coil never had to remove that. It is worth about 1% of the total — small, exact, and free to include.
Step 5 — the split
qsensible = ṁda × ( 0.240 + 0.444 W1 ) × ( t1 − t2 )
qlatent = ṁda × ( W1 − W2 ) × ( 1061 + 0.444 t2 )
SHR = qsensible ÷ qtotal
qlatent = ṁda × ( W1 − W2 ) × ( 1061 + 0.444 t2 )
SHR = qsensible ÷ qtotal
Cool the air at constant moisture content first, then remove the moisture at constant temperature. Substitute the enthalpy equation into both legs and the two parts sum to exactly the total enthalpy difference — no residual, no fudge. The sensible heat ratio is then just their ratio, which is also, as CED Engineering's psychrometrics course puts it, “defined by the slope of the line connecting the two points” on a psychrometric chart.
One subtlety worth knowing. The specific heat in the sensible leg is 0.240 + 0.444 W, not a flat 0.240. Moist air holds water vapour, and water vapour has its own heat capacity. At a typical return-air humidity ratio that is about 2% more sensible capacity than the plain dry-air figure gives — and using the flat 0.240 is what makes the sensible and latent parts fail to add up to the total.
The 1.08, 0.68 and 4.5 constants — what they are and when they lie
Every technician meets these before they meet a psychrometric equation:
qsensible = 1.08 × CFM × ΔT
qlatent = 0.68 × CFM × Δgrains (or 4840 × CFM × ΔW)
qtotal = 4.5 × CFM × Δh
qlatent = 0.68 × CFM × Δgrains (or 4840 × CFM × ΔW)
qtotal = 4.5 × CFM × Δh
They are not magic numbers. CED Engineering's course derives all three in four lines, and it is worth seeing because the derivation is also the list of assumptions:
lb/h = CFM × 0.075 lb/ft³ × 60 min/h = CFM × 4.5
sensible = 4.5 × 0.24 = 1.08
latent = 4.5 × 1054 ÷ 7000 = 0.68 per grain
sensible = 4.5 × 0.24 = 1.08
latent = 4.5 × 1054 ÷ 7000 = 0.68 per grain
So the whole family rests on one number: 0.075 pounds per cubic foot, which is 13.333 cubic feet per pound. That is dry air, at sea level, at around 70 °F. Three assumptions, and a job site rarely satisfies any of them.
| Air condition | Real specific volume | Error in the 4.5 constant |
|---|---|---|
| 80 °F / 67 °F WB, sea level (AHRI rating condition) | 13.85 ft³/lb | 3.7% high |
| 75 °F / 62 °F WB, sea level | 13.67 ft³/lb | 2.5% high |
| 78 °F / 65 °F WB, 2,500 ft | 15.12 ft³/lb | 11.8% high |
| 80 °F / 67 °F WB, 5,280 ft (Denver) | 16.90 ft³/lb | 21.1% high |
Specific volumes computed from ASHRAE eq (26) at the barometric pressure eq (3) gives for each elevation. The error column is how much more air the 4.5 constant thinks you are moving than you actually are.
Read that bottom row again if you work at altitude. In Denver, the standard-air constants overstate mass flow by more than a fifth. Every BTU figure that comes out of them is overstated by the same fraction, and a system that looks like it is making rated capacity may be well short of it. The calculator above takes elevation as an input for exactly this reason.
None of this makes the constants useless. For a load estimate, where the inputs are assumptions anyway, they are fine and they are fast. For a measurement you are about to make a warranty claim or a replacement recommendation with, they are the wrong tool.
What the sensible heat ratio is actually telling you
SHR is the fraction of the coil's total output that goes into lowering temperature. An SHR of 0.80 means four-fifths of the work is sensible and a fifth is dehumidification. An SHR of 1.00 means the coil is bone dry and removing no water at all.
Two things set it, and they pull in opposite directions:
- How wet the entering air is. Humid return air condenses more readily on a given coil surface, which raises the latent share and pushes SHR down.
- How fast the air is moving. More airflow across the same coil means less contact time and a warmer coil surface, which condenses less water and pushes SHR up. Less airflow does the reverse — and taken far enough, freezes the coil.
That second one is the lever a technician actually has, and it is why blower speed is a dehumidification control and not just an airflow control.
This page will not tell you what SHR you ought to have. There is no published band with a test method behind it, and there cannot be a general one: the same coil at the same airflow legitimately reads differently in Phoenix and in Houston, because the entering air is different. What the page gives you instead is the measured figure, the AHRI rating condition it can be compared against, and the arithmetic to re-run it after you change something. A number that moves the way you predicted when you changed blower speed is worth more than a number that matched a table.
What a nameplate rating actually is
ANSI/AHRI Standard 210/240 rates cooling capacity at a specific condition, and it is printed in the standard's own test table:
| Test | Entering indoor unit | Entering outdoor unit |
|---|---|---|
| A test (required) | 80.0 °F DB / 67.0 °F WB | 95.0 °F DB / 75.0 °F WB |
| B test (required) | 80.0 °F DB / 67.0 °F WB | 82.0 °F DB / 65.0 °F WB |
Reproduced from the cooling test conditions table of ANSI/AHRI Standard 210/240-2017.
If your return air is at 75 °F and 50% relative humidity on a mild day, the coil has less to work with than the rating condition gives it, and a measured capacity below the nameplate is expected rather than alarming. The calculator prints your conditions next to AHRI's so the mismatch is visible, and it deliberately does not apply a correction factor — there is no general one, and the manufacturer's expanded performance tables exist precisely because there isn't.
Where the error really lives
Three inputs carry almost all of the uncertainty, and they are not equally to blame.
- Airflow, by a distance. It multiplies every single BTU figure on the page. A traverse that is 10% out makes the total, the sensible and the latent all 10% out, in the same direction. It does not move the sensible heat ratio, though — SHR is a ratio of two numbers that both scale with airflow, so it survives a bad CFM completely intact. That is a genuinely useful property: if you do not trust your airflow, you can still trust your SHR.
- The supply wet bulb. Supply air off a wet coil sits within a couple of degrees of saturation, so the entire latent calculation depends on a small difference between two nearly equal numbers. Half a degree of error on that reading typically moves the latent capacity by something like a fifth. The table under the results re-runs the whole calculation across a three-degree window so you can see it for your own case rather than taking that on trust.
- Probe placement. A return probe downstream of a leaky return drop is reading a blend of house air and attic air. A supply probe in line of sight of a cold coil reads low by radiation. Neither shows up as an obviously wrong number — they just quietly move the answer.
A free consistency check. The moisture removal figure is in pints per hour, and condensate is a thing you can see. Put a container under the drain for ten minutes and multiply. If the calculator says three pints an hour and the drain yields roughly half a pint in ten minutes, your wet bulbs are good. If the drain is dry, something in the humidity measurement is wrong — or the pan is draining somewhere it should not be.
One honest caveat about the numbers
ASHRAE publishes its psychrometrics twice over. There is a set of perfect-gas relations — the equations above, the ones intended for calculation — and there is Table 2 and the printed psychrometric chart, which are computed from an exact formulation carrying an enhancement factor that corrects for dissolved gases, the pressure effect on liquid water, and vapour–air interaction.
The two disagree slightly. The enhancement factor runs about 1.004 to 1.005 over normal HVAC temperatures, so a humidity ratio read off a chart at saturation comes out roughly 0.4% higher than the one these equations produce. This calculator uses the perfect-gas relations, as PsychroLib and essentially every field tool does, because those are the equations ASHRAE prints for computing with. Carried through to a delivered-capacity answer the difference is at most about 0.3%, and under 0.003 on the sensible heat ratio.
That is far smaller than the airflow measurement it gets multiplied by, which is why nobody bothers with it. But it is the reason a chart reading and a computed figure differ in the third significant figure, and if you have been chasing that discrepancy, that is where it comes from.
A second, older mismatch. ASHRAE revised three constants between the 2001 and 2009 editions of the handbook: the molecular mass ratio went from 0.62198 to 0.621945, and the water-vapour terms from 1.805 and 2.381 to 1.86 and 2.326. This page uses the current values. If you are checking against a pre-2009 textbook or a chart printed from one, expect a disagreement of up to about 0.05% on enthalpy.
Frequently asked questions
What is sensible heat ratio?
Sensible heat ratio is the sensible cooling capacity of a coil divided by its total cooling capacity. An SHR of 0.75 means three-quarters of the coil's output is lowering air temperature and one-quarter is condensing water out of it. An SHR of 1.0 means the coil is dry and doing no dehumidification at all.
How do you calculate sensible heat ratio from measurements?
Measure airflow in CFM and the dry-bulb and wet-bulb temperatures entering and leaving the coil. Convert each pair to a humidity ratio and an enthalpy using the ASHRAE psychrometric equations at your local barometric pressure, work out the dry-air mass flow from the airflow and the specific volume, then multiply the mass flow by the enthalpy difference for total capacity and by the temperature difference and moist-air specific heat for sensible capacity. SHR is the second divided by the first.
Why do I need a wet-bulb temperature and not just dry bulb?
A dry-bulb reading only sees sensible heat. Any heat the coil removes by condensing water out of the air — the latent capacity — is invisible to a thermometer. On a humid day that can be a quarter or more of the total, so a dry-bulb split alone cannot tell you what the equipment is actually delivering.
What does the 1.08 constant in HVAC actually mean?
It is 4.5 multiplied by 0.24. The 4.5 comes from standard air at 0.075 pounds per cubic foot times 60 minutes per hour, which converts CFM to pounds per hour, and 0.24 is the specific heat of dry air. It therefore assumes dry air, at sea level, near 70 °F. At 5,280 feet the real air is around 21% less dense and the constant overstates capacity by that much.
Does a bad airflow measurement ruin the sensible heat ratio?
No. Airflow scales the sensible and the total capacity by the same factor, so it cancels out of their ratio completely. A 10% error in CFM makes every BTU figure 10% wrong but leaves the sensible heat ratio exactly where it was. The BTU numbers need good airflow; the ratio does not.
Why is my latent capacity coming out negative?
Because the supply air computed wetter than the return air, which a cooling coil cannot do. In practice it is a measurement problem: a supply wet-bulb probe sitting in condensate or with a fouled wick, readings taken minutes apart while conditions moved, or a humidifier downstream of the coil and upstream of the probe.
Should I compare my measured capacity to the nameplate?
Only as a sanity check. AHRI 210/240 rates capacity with 80 °F dry bulb and 67 °F wet bulb entering the indoor coil and 95 °F entering the outdoor unit. Cooler or drier return air than that legitimately gives less capacity. A reading below nameplate is a prompt to investigate, not a fault in itself, and the manufacturer's expanded performance data for your actual conditions is what settles the question.
Does elevation really matter for cooling capacity?
Yes, more than most corrections. Capacity depends on the mass of air moved, not its volume. At 2,500 feet the same CFM carries about 12% less air by weight than at sea level, and at 5,280 feet about 21% less. A calculation done with sea-level constants at altitude is optimistic by that amount.
What is the condensate enthalpy credit?
Water leaving the coil as liquid at the supply air temperature carries some enthalpy away with it, and the coil never had to remove that part. ASHRAE's cooling-and-dehumidification equation subtracts it from the total: the mass of water condensed multiplied by its liquid enthalpy. It is worth roughly 1% of total capacity — small, but exact and free to include.
Can I use relative humidity instead of wet bulb?
Yes, and the calculator accepts either. Both are converted to a humidity ratio through the same ASHRAE equations, so neither route is more correct. In practice wet bulb is often the better field measurement for supply air, because supply air off a wet coil sits at 90% relative humidity or higher and that is where hygrometers are least accurate.