AC Tonnage Calculator
Engineering Specification Sheet
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Generated: August 10, 2026
AC Tonnage Calculator
Decode the nominal tonnage of an air conditioner or heat pump from its model number. Handles the published codes that the divide-by-12 shortcut gets wrong.
The Nameplate
The capacity code is not always the code ÷ 12.
Trane prints 037 for a 3 ton. Divide that by 12 and you get 3.08 tons, which no manufacturer builds. Goodman does the same with 019, 025, 031, 037 and 043; Lennox with 041, 047 and 059. And a Carrier 24ACC636A003 opens with the digits 24 — read left to right, a 3 ton becomes a 2 ton.
Use the MODELnumber from the outdoor unit’s data plate, not the serial. The model encodes what the unit is; the serial encodes when it was built. Dashes, spaces and case do not matter.
Every brand pattern is tried. If two brands disagree about the capacity, this refuses to pick rather than guess.
Nominal capacity
Derivation
Read the naive way, 37 ÷ 12 would be 3.08 tons — a size no manufacturer builds. Trane / American Standard published codes governs here, and the answer is 3 tons.
A nominal tonnage is a marketing size, not a measured output. Rated capacity is established at AHRI conditions, and what the system actually delivers depends on the matched indoor coil, airflow, charge and altitude.
Where each brand prints the capacity code
The field sits at a different offset in every manufacturer’s nomenclature, which is why a single left-to-right scan cannot be right for all of them. Each row states what its position rests on — a two-source row was corroborated by two independent publications, a single-source row by one. Brands whose position could not be corroborated are not listed at all; they fall through to a scan that says it did not recognise the brand.
| Brand family | Example | Reads | Basis |
|---|---|---|---|
| Goodman / Amana2 documented families | GSX160361 | 3 toncode 036 | two sourcesGoodman's literature library plus the furnace.direct decoder guide agree the three-digit group is Btu/h in thousands. The five odd codes rest on one source plus the snap invariant. |
| Trane / American Standard1 documented family | 4TTR6037J1000AA | 3 toncode 37 | two sourcesPartsTown's Trane model guide and intrysys.com agree that symbols 7-8 are the nominal capacity in thousands of Btu/h and that position 6 is not part of the code. The seven odd codes rest on one source plus the snap invariant. |
| Lennox3 documented families | XC13-036-230 | 3 toncode 036 | two sourcesThe Lennox XC13 Engineering Handbook prints the nominal cooling capacity table directly, 018 = 1.5 tons through 060 = 5 tons. The 041/047/059 codes on the newer line rest on one source plus the snap invariant. |
| Carrier / Bryant / Payne4 documented families | 24ACC636A003 | 3 toncode 36 | one sourcePosition published by intrysys.com only, corroborated only by the decoded values landing on the standard ladder. The Carrier two-digit read is the weakest pattern in this file: a 24ACC6 model string opens with the digits 24, which a naive scan misreads as 2 tons. |
| Rheem / Ruud2 documented families | RA1642AJ1NA | 3.5 toncode 42 | one sourceRead by documented pattern from model strings rather than from a fetched Rheem nomenclature sheet. Every shipped example lands on the standard ladder, which is a consistency check and not a second source. |
| Gibson / Frigidaire / Maytag / Westinghouse / Intertherm / Miller1 documented family | ET4BE-036K | 3 toncode 036 | one sourceGrouped and published by intrysys.com as one shared suffix scheme across the Nortek house brands, labelled capacity-only. No manufacturer sheet was fetched. |
| Ductless mini-split (Mitsubishi / Fujitsu / LG / MrCool)3 documented families | MSZ-GL12NA | 1 toncode 12 | two sourcesThe Btu-class convention is stated by intrysys.com and by the general decoder literature. LG's three-digit hundreds-of-Btu form is single-source and is why `codeUnit` exists as a field. |
Capacity code cross-reference
The card to print and leave in the van. Rows are nominal sizes, columns are brand families, cells are the code that family prints. A + marks a size where the brand also publishes a code the naive division gets wrong.
| Size | Btu/h | Goodman | Trane | Lennox | Carrier | Rheem | Gibson | Ductless mini-split |
|---|---|---|---|---|---|---|---|---|
| 1 ton | 12,000 | 012 | 12 | 012 | 12 | 12 | 012 | 12 |
| 1.5 ton | 18,000 | 018+ | 18+ | 018 | 18 | 18 | 018 | 18 |
| 2 ton | 24,000 | 024+ | 24+ | 024 | 24 | 24 | 024 | 24 |
| 2.5 ton | 30,000 | 030+ | 30+ | 030 | 30 | 30 | 030 | 30 |
| 3 ton | 36,000 | 036+ | 36+ | 036 | 36 | 36 | 036 | 36 |
| 3.5 ton | 42,000 | 042+ | 42+ | 042+ | 42 | 42 | 042 | 42 |
| 4 ton | 48,000 | 048 | 48+ | 048+ | 48 | 48 | 048 | 48 |
| 5 ton | 60,000 | 060 | 60+ | 060+ | 60 | 60 | 060 | 60 |
How the code is found.The model number is upper-cased and every character that is not a letter or a digit is stripped, which removes the hyphens Lennox, Mitsubishi and MrCool print. Each brand’s patterns are then tried in order, anchored to the start of the string so a capacity field can never be picked up out of the middle of a serial number, and the first match wins. When the brand is set to auto-detect and two brands match with different readings, this page returns nothing and names both, because there is no honest way to choose between them from here.
Where the brands disagree with divide-by-twelve. Trane and American Standard print 19, 25, 31, 37, 43, 49 and 61. Goodman and Amana print 019, 025, 031, 037 and 043. Lennox’s newer line prints 041, 047 and 059. Every one of those is exactly 1,000 Btu/h off its nominal size, which is a useful property: the plus-or-minus 1,000 snap this tool applies reproduces every one of those published values without consulting the table, so the two act as independent checks on each other. Mini splits are sold by Btu class and print it directly — 09, 12, 15, 18, 24, 30, 36 — and LG prints hundreds rather than thousands, so its 120 is 12,000 Btu/h and not 120,000.
Why there is no square-footage box on this page. Because it would turn a sourced tool into an unsourced one. There is no authoritative table behind Btu-per-square-foot or tons-per-square-foot; the climate-zone ranges that populate every competing calculator vary by source, and published criticism puts the rule of thumb at 40 to 100 percent oversizing. The honest method is ACCA Manual J, which is proprietary and which real load software is licensed to perform. So this page answers the question that has a verifiable answer — what does the equipment say it is — and the sizing question is handled in writing instead, where the uncertainty can be described rather than hidden behind a number.
What a nominal tonnage is not. It is a marketing size, not a measured output. Rated capacity is established at AHRI test conditions, and what a system actually delivers depends on the matched indoor coil, the certified AHRI combination, airflow, refrigerant charge and altitude. A furnace model number carries a code that looks identical and means gas input, with no tonnage at all — this tool refuses those rather than converting them. For the air side of the same job see the duct size calculator, and for what a replacement saves, the SEER2 savings calculator.
How this works
A ton of air conditioning is a rate of heat removal, not a weight. One ton is 12,000 Btu/h by definition — the heat absorbed by a short ton of ice melting over 24 hours — so once you know a unit's capacity in Btu/h, the tonnage is a single division. Manufacturers encode that capacity directly in the model number, which is why the data plate on the outdoor unit answers the question and the serial number does not.
The catch is that the shortcut everyone repeats, take the capacity digits and divide by twelve, is right for most codes and quietly wrong for a documented minority. Trane and American Standard print 037 for a three ton; divide that by twelve and you get 3.08 tons, a size no manufacturer builds. Goodman and Amana do the same with 019, 025, 031, 037 and 043, and Lennox's newer line with 041, 047 and 059. On those codes the brand's own published table governs.
The second trap is position. The capacity field sits at a different offset in every manufacturer's nomenclature, so a scan that takes the first plausible number in the string reads a Carrier 24ACC636A003 as a two ton when it is a three. This tool matches each brand's documented pattern instead, shows which digits it took, and where the brand is unknown and two candidate codes are present, it declines to choose rather than guess.
Worked example
A Trane condenser with 4TTR6037J1000AA on the data plate — the code the divide-by-twelve shortcut gets wrong.
The unit is a 3 ton. Read the naive way, 37 divided by 12 gives 3.08 tons — a size that does not exist, which is the tell that the published table governs this code.
A Carrier condenser, 24ACC636A003, where the model number opens with the digits 24.
A 3 ton, not the 2 ton a left-to-right scan would report. The leading 24 is the Carrier product-series prefix; the capacity field sits after the family group.
A Mitsubishi ductless head, MSZ-GL12NA, where the hyphens and the Btu class both need handling.
A 12,000 Btu/h head, which is one ton. Ductless equipment is sold and specified by Btu class rather than tonnage, so 12,000 Btu/h is the number to quote back.
Frequently Asked Questions
How do I find the tonnage of my AC unit from the model number?↓
Find the model number on the data plate of the outdoor unit — not the serial number, which only records when it was built. Inside the model number is a two- or three-digit capacity code giving nominal capacity in thousands of Btu/h: 018 is 1.5 tons, 024 is 2, 030 is 2.5, 036 is 3, 042 is 3.5, 048 is 4 and 060 is 5. One ton is 12,000 Btu/h, so the code divided by 12 gives the tonnage for most brands.
Why is a Trane 037 a 3 ton and not 3.08 tons?↓
Because 3.08 tons is not a size anyone manufactures. Trane and American Standard publish a capacity code table that prints 19, 25, 31, 37, 43, 49 and 61 for 1.5 through 5 tons, and on those codes the published table governs rather than the arithmetic. Goodman and Amana print 019, 025, 031, 037 and 043 on the same principle, and Lennox prints 041, 047 and 059. Every one of them is exactly 1,000 Btu/h away from its true nominal size.
Can I work out what size unit I need from my square footage?↓
Not reliably, and this tool deliberately will not do it. There is no authoritative table behind Btu-per-square-foot or tons-per-square-foot — the climate-zone ranges published on calculator sites vary by source, and criticism of the practice puts the naive rule of thumb at 40 to 100 percent oversizing. The method the industry recognises is an ACCA Manual J load calculation, which accounts for insulation, orientation, window area and infiltration. An oversized system short-cycles, removes less humidity and wears out sooner, so the rule of thumb is not a conservative shortcut.
Should I decode the outdoor unit or the indoor unit?↓
The outdoor unit. Cooling capacity is set by the condenser or heat pump, and the delivered capacity of the system is set by the AHRI-certified combination of that outdoor unit with a specific indoor coil and air handler. An air handler or coil model number carries its own capacity code, which describes that component rather than the system.
Why does my furnace model number not give a tonnage?↓
Because a furnace has no tonnage. A furnace capacity code looks identical to a cooling code but means heating input in thousands of Btu/h — a Goodman GMVC960603BN reads 060, which is 60,000 Btu/h of gas input and not a five ton. Reading a furnace model as a tonnage is the most common error on this subject, so this tool identifies furnace nomenclature and refuses to convert it.
What do mini-split and LG model numbers mean?↓
Ductless equipment is sold by Btu class rather than tonnage, and the class number appears directly in the model: 09, 12, 15, 18, 24, 30 and 36 are 9,000 through 36,000 Btu/h. A Mitsubishi MSZ-GL12NA is a 12,000 Btu/h head, or one ton. LG is the exception that proves the need for care — it prints hundreds rather than thousands, so an LSU120HSV5 is 12,000 Btu/h and not 120,000.
What happens if the model number is not recognised?↓
The tool says so rather than guessing. If no brand pattern matches, it scans the string for a standard capacity code and returns one only when exactly one is present, flagged as a scan rather than a decode. If the string contains two plausible codes it returns nothing and names both, because which one is the capacity depends on a nomenclature it does not have. A confident wrong tonnage with a citation under it is worse than no answer.
Learn more
Manual J Load Calculations Explained: What Contractors Actually Need to Know
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Why Square-Footage HVAC Sizing Rules Are Wrong — and What Manual J Actually Does
The 500-square-feet-per-ton rule can miss by 40 to 100 percent. Here is where it comes from, why oversizing makes comfort worse, and what a real load calculation accounts for.
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