SizingKitEquipment selection
AC tonnage calculator
Dividing a cooling load by 12,000 gives tons in a second; the part that takes judgment is that residential air conditioners exist in only seven sizes, half a ton apart until the top of the range, and ACCA Manual S will not let you round up past 115% of the load. This free tool checks every one of those seven sizes against your load, marks which the window allows, and prints how much of each hour every size would actually run. No signup, nothing uploaded — it works out in the browser tab, which is where a phone in a crawlspace needs it to.
- 100% free
- No signup
- 7 nominal sizes
- Manual S 90–115%
- 400 CFM per ton
The load, and what it is being selected against
Total cooling load at the design conditions — the figure a load calculation returns, not a square-foot guess and not the size of what is there now.
Where the load comes from: btu calculator.
Manual S allows 115% here. Total capacity against the total cooling load, at the design conditions. The upper bound is about humidity: a machine much above the load stops before it has dehumidified.
Default 400 — The airflow a nominal ton of residential cooling capacity is rated at: AHRI 210/240 rates split systems at a stated airflow, and 400 CFM/ton is the industry nominal against which a rating is quoted. 350 CFM/ton in humid climates where latent removal matters, up to 450 in dry climates where it does not. ACCA Manual S sets airflow from the required sensible/latent split, not from a nominal figure, and the equipment's own expanded performance data is what a design should use.
Which of the seven sizes the load allows
LOAD IN TONS
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LOAD IN KILOWATTS
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| Nominal | BTU/h | Of load | Runs | Manual S |
|---|---|---|---|---|
| Enter a cooling load and the seven sizes are checked against it. | ||||
ACCA Manual S (Residential Equipment Selection), whose limits on how far selected capacity may exceed the Manual J load are the reason a load calculation is worth doing at all. Manual S has been revised and the exact bounds and their conditions differ between editions and between climates — the humid-climate rules on latent capacity are stricter than the numbers here suggest, and there are separate provisions for variable-capacity equipment, which can modulate below its rated output and is not well described by a single ratio. Check the current edition before quoting a limit as a requirement. The limits apply to capacity at the design conditions from the manufacturer's expanded data, not to nominal tonnage.
US residential split-system nominal cooling sizes, as listed in the AHRI Directory of Certified Product Performance and as encoded in manufacturers' model numbers: the three-digit group is nominal capacity in MBH, so 018 is 18,000 BTU/h and 060 is 60,000 BTU/h. Nominal, not rated. A nominal size is a label for a product line, not a measured capacity: the AHRI-certified capacity of a given indoor/outdoor combination at the rating conditions is published per combination and is what Manual S is applied to. Most residential lines have no 054, so the step from 4 to 5 tons is a full 12,000 BTU/h wide, and light commercial equipment continues above 5 tons in sizes this list does not carry. A heat pump selected here still has to be checked against the heating season, where the same machine gets a heat pump sizing calculator and a balance point; and the tonnage itself is only a rate, worked through on the btu to tons calculator.
How to pick a tonnage a Manual S review will accept
The division is the easy third of it.
Put in the total cooling load, not the floor area
The figure this page wants is the whole-house cooling load at your design conditions, in BTU/h, tons, kW, MBH or watts. Square feet is not an input here on purpose: two houses of the same area in the same town routinely differ by a factor of two, and a tool that accepts area has to invent the difference.
Say whether it is an air conditioner or the cooling side of a heat pump
The two get different ceilings — 115% and 125% — because a heat pump is usually being asked to satisfy a heating load as well and Manual S gives that conflict somewhere to go. The window recalculates and the ladder re-marks as soon as you switch.
Read down the ladder, not just at the highlighted row
The row that fits is marked, but the ones that do not are the useful part: they show by how much, and in which direction. Where the load falls in the gap between two sizes and neither is allowed, that is the honest answer — the selection needs two-stage or variable-capacity equipment chosen from its own performance data, not a rounder number.
Technical specifications
| Sizes checked | 7 — 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons, which is 18,000 to 60,000 BTU/h of nominal capacity. Most residential lines have no 054, so the last step is twice as wide as the others. |
|---|---|
| Step between sizes | 6,000 BTU/h up to 4 tons, then a single 12,000 BTU/h jump from 4 tons to 5. A load landing inside that jump is the case single-stage equipment cannot answer cleanly. |
| Manual S window, air conditioner | 90% to 115% of the total cooling load at the design conditions. The ceiling is about moisture, not energy — it is the only sizing bound in residential HVAC set by comfort rather than capacity. |
| Manual S window, heat pump on cooling | 90% to 125%. The extra 10 points exist because the heating side of the same machine usually asks for more capacity than the cooling side does, and the compromise has to fall somewhere. |
| Run time at the ceiling | A selection at 115% of the load runs 87% of the hour at design. At 167% — a 5-ton unit on a 3-ton load — it runs 60%, and the coil spends the other 40% of every hour drying out and giving its condensate back to the house. |
| Airflow assumed | 400 CFM per nominal ton, editable between 200 and 600. Three tons at the default is 1,200 CFM, or 566 L/s. |
| Indoor design condition | 75 °F dry bulb at 50% relative humidity — the ACCA Manual J default the load handed to this page should have been calculated at. |
| What the ladder is not | Nominal capacity is a product-line label. A design is signed off against the certified capacity of the specific indoor and outdoor combination at your own design conditions, read out of the manufacturer's expanded performance data — and that figure moves with indoor wet bulb and outdoor dry bulb, which a nominal number cannot. |
Frequently asked questions
What actually goes wrong if I put in a bigger unit than the load?
The house gets cold and stays damp. An air conditioner removes moisture only while the coil is wet and the air is moving across it, and a coil takes several minutes of continuous running to get there — so a machine that satisfies the thermostat in six minutes has been dehumidifying for two of them. Worse, the condensate clinging to an idle coil evaporates back into the house on the next fan cycle, so an oversized system can return moisture it had already removed. That is why the ladder on this page carries a run-time column: the percentage is the mechanism, and 115% is where Manual S decides it has gone too far.
Why does the ladder stop at 5 tons?
Because that is where residential split systems stop. Above 60,000 BTU/h the market moves to light commercial packaged and split equipment, in sizes this ladder does not carry and against a different set of constraints — three-phase power, curb dimensions, economizers, a roof that has to take the weight. A house whose calculated load lands above 5 tons is more often a house with a load calculation worth re-checking than one that needs commercial equipment, and the second thing to check after that is whether it should be two systems rather than one.
My load sits between two sizes and the ladder says neither is allowed. Now what?
Take it as a real result rather than a rounding problem. The 6,000 BTU/h step between nominal sizes is wider than the Manual S window at some loads, and single-stage equipment simply has nothing that fits — which is precisely the case two-stage and inverter-driven equipment exists for, because it can modulate down to a fraction of its rated output and hold the coil wet at part load. Select it from its own expanded performance data at your design conditions rather than from a nominal label, and check the low-stage capacity as well as the high.
Why does a heat pump get 125% when an air conditioner only gets 115%?
Because the same box has to do two jobs and the jobs disagree. In most of the US the heating load is larger than the cooling load, so a heat pump sized to satisfy winter without leaning on resistance heat is bigger than the summer wants; Manual S widens the cooling allowance to let some of that conflict be resolved on the cooling side rather than all of it on the auxiliary heat. Where the winter is what governs, the number that decides the trade-off is the balance point on the heat pump sizing calculator, not this window.
Should the selection be made on total capacity or sensible capacity?
Both, and the sensible one is the one that fails. Manual S has you check total capacity against the total load and sensible capacity against the sensible load, at the design conditions, from the equipment's expanded performance data — and equipment that passes the total check can still be short on sensible, or long on it and therefore short on latent. A machine long on sensible cools the air fast and dries it slowly, which is the same complaint as oversizing arriving by a different route. A total-only check, which is all a nominal tonnage can support, will not see it.
Does the airflow figure change which size I should buy?
No, but it changes what that size does. Nominal capacity is quoted at a rating airflow near 400 CFM per ton, and moving less air across the same coil drives the coil colder — less sensible capacity, more latent, which is what you want in a humid climate and why 350 CFM per ton is common there. More air does the opposite and suits a dry climate. The tonnage on this page comes out of the load; the airflow decides how that tonnage splits between drying the air and cooling it, and it is a field here rather than a stated fact because the right value moves with the climate.
Can I use this to size a ductless mini-split?
Only as a first look. Ductless equipment is inverter-driven, so its useful capacity is a range rather than a point — its minimum and maximum output are both published, and both matter — which makes the Manual S window as a pair of fixed percentages the wrong shape of test, and Manual S itself carries separate provisions for equipment that can modulate below its rated output. Multi-head systems add a second problem the ladder cannot see: the outdoor unit's rated total is not the sum of the heads that may be connected to it, so each room's load has to be satisfied against the manufacturer's combination table rather than against the head's own label.
About half-ton steps, and the ceiling nobody expects
Almost everyone arriving at a tonnage question expects the answer to have a floor and no roof: get enough capacity, and any surplus is insurance against a hot week. Cooling is the one part of building services where that instinct is exactly backwards, and the reason is water rather than heat. An air conditioner does two things at once — it drops the air temperature and it condenses moisture out of it — but only the first happens immediately. The coil has to get cold and wet before it starts pulling water out of the air stream, which takes several minutes of continuous operation, and a thermostat satisfied before that point ends the cycle with the sensible job done and the latent job barely started. Run the arithmetic in the ladder above and it stops being an opinion: capacity at 167% of the load runs 60% of the hour, and the 40% it spends off is when the wet coil hands its condensate back to the house.
ACCA Manual S is the document that turns that into a rule. Its sequence is strict and it is worth knowing in order, because most sizing arguments are really arguments about which step was skipped: run a Manual J load calculation at your own design conditions; take the sensible and latent loads out of it separately; then select equipment from the manufacturer’s expanded performance data at those same conditions — not from the nominal tonnage on the carton, which was measured somewhere else entirely. Only then do the percentage bounds apply, and they apply to the capacity you read out of that data. The window in this page is the last step of the four, and it is the only one a web page can do for you; the load is a btu calculator at best and a manual j calculator properly, and the expanded data is the manufacturer’s.
The half-ton ladder itself is an artifact of manufacturing rather than of physics, and it is why the window bites. Compressors are built in a handful of displacements per product line, so seven sizes cover the whole residential market, and the 6,000 BTU/h gap between adjacent ones is coarse enough that a load calculation landing at 41,000 BTU/h has 42,000 available at 102% and nothing else inside the window at all. Understanding the raw conversion first helps here — what a ton is and how it relates to kW and kcal/h is the btu to tons calculator — and if the box in question also heats, the winter has its own answer on the heat pump sizing calculator. None of this is a stamped design: a permit that asks for equipment selection is asking for a signed Manual J and Manual S from someone who has seen the house, and what the installed machine is actually doing is measured later at the gauges, on the subcooling calculator.
Where the selection is worked out
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The ladder is recomputed on every keystroke in the page itself, so a load you would rather not send to a manufacturer’s sizing portal never leaves the device — and the marked-up ladder prints straight from the browser for the job folder.