HVAC BTU & Tonnage Calculator
Btu/h, tons and kW from airflow, water flow or room size — solve any way, see the working.
Calculate
One room, from the ENERGY STAR room air-conditioner chart (8-ft ceilings).
Heat carried by air: q = ρ·cp·V̇·Δt — 1.08 × cfm × Δt for standard air.
Heat carried by water or another liquid: q = 500 × gpm × Δt, or ρ·cp·V̇·Δt.
1 ton of refrigeration = 12,000 Btu/h = 3.517 kW; 1 MBH = 1,000 Btu/h.
Common sizes that cover it
| Type of unit | Size | Above the need |
|---|
Typical nominal sizes on sale. Compare the rated cooling capacity on the label, which can differ from the nominal size; a much bigger unit than needed cycles on and off and removes less moisture.
ENERGY STAR sizing chart
| Area (sq ft) | Area (m²) | Btu/h |
|---|---|---|
| 100 up to 150 | 9.3–13.9 | 5,000 |
| 150 up to 250 | 13.9–23.2 | 6,000 |
| 250 up to 300 | 23.2–27.9 | 7,000 |
| 300 up to 350 | 27.9–32.5 | 8,000 |
| 350 up to 400 | 32.5–37.2 | 9,000 |
| 400 up to 450 | 37.2–41.8 | 10,000 |
| 450 up to 550 | 41.8–51.1 | 12,000 |
| 550 up to 700 | 51.1–65.0 | 14,000 |
| 700 up to 1,000 | 65.0–92.9 | 18,000 |
| 1,000 up to 1,200 | 92.9–111.5 | 21,000 |
| 1,200 up to 1,400 | 111.5–130.1 | 23,000 |
| 1,400 up to 1,500 | 130.1–139.4 | 24,000 |
| 1,500 up to 2,000 | 139.4–185.8 | 30,000 |
| 2,000 up to 2,500 | 185.8–232.3 | 34,000 |
Then −10 % for a heavily shaded room or +10 % for a very sunny one, +600 Btu/h for each person beyond two and +4,000 Btu/h for a kitchen. Hot climate, top floor or a tall ceiling? Try the AC tonnage calculator’s heat-gain method. A whole house or a ducted system needs a room-by-room ACCA Manual J load calculation.
Air on each side
| Property, unit | Entering | Leaving |
|---|
In every unit
| Unit | Value |
|---|
How it was calculated
Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the HVAC BTU & Tonnage Calculator
Work out heating or cooling capacity in Btu/h, tons and kW four ways. Room size reads the ENERGY STAR room air-conditioner chart with its adjustments for sun, people and kitchens, and shows the common unit sizes that cover the need. Airflow uses the sensible-heat equation q = 1.08 × cfm × Δt — or ρ·cp·V̇·Δt for the real air at your elevation — and, with wet-bulb or RH readings, the total and latent heat from the enthalpy change. Water flow uses q = 500 × gpm × Δt, or water’s own density and specific heat at its temperature (IAPWS-IF97), for chilled-water and hot-water coils. Airflow and water flow can each be solved for the capacity, the flow or the temperature difference, and Convert turns Btu/h, MBH, tons, kW and kcal/h into each other.
Every result shows its working. These are estimates for checking and sizing one room, coil or pipe — a whole house or a central system needs a room-by-room load calculation such as ACCA Manual J.
How to use it
- Pick Room size, Airflow, Water flow or Convert, and US or Metric units; the capacity, flow and elevation boxes also have their own unit menus.
- Room size: enter the room’s length and width (or its floor area), how sunny it is, how many people usually use it and whether the unit is for a kitchen.
- Airflow or Water flow: choose what to find — the capacity, the flow or the temperature difference — and fill in the other two. Temperatures can be the two readings (entering and leaving) or just the difference.
- For a cooling coil, tick Include humidity and enter the wet-bulb or RH readings on both sides to get the total capacity, the latent part and the water condensed. Above about 2,000 ft, choose Air at your elevation so the 1.08 factor is corrected.
- Read the headline and the working under it, then copy or download the result.
Examples
ENERGY STAR row 150 up to 250 sq ft: 6,000 Btu/h = 0.5 ton (1.76 kW) → a 6,000 Btu/h window AC.
9,000 − 10 % + 2 × 600 = 9,300 Btu/h → the next common window size is 10,000 Btu/h; a mini-split or a 1-ton unit is 12,000 Btu/h.
1.08 × 1,200 × 20 = 25,920 Btu/h sensible = 2.16 tons (7.6 kW).
Total 4.5 × cfm × Δh = 4.5 × 1,200 × 7.662 Btu/lb = 41,375 Btu/h (3.45 tons, 348 cfm per ton); sensible 28,512 Btu/h, latent 12,863 Btu/h, sensible heat ratio 0.69; about 5.1 L (11.3 lb) of condensate an hour.
Temperature rise = 80,000 ÷ (1.08 × 1,200) = 61.7 °F — compare it with the rise range on the furnace’s rating plate.
Air at 83.4 kPa and 75 °F weighs 0.0611 lb/ft³, so the factor is 0.881 instead of 1.08: 21,135 Btu/h, 18 % less than at sea level.
500 × 24 × 10 = 120,000 Btu/h = 10 tons, 2.4 gpm per ton. With IAPWS water properties at 49 °F: 120,420 Btu/h.
The rule gives 100,000 Btu/h; water at 170 °F has a factor of 488.3, so 97,666 Btu/h (2.3 % less).
At 9.5 °C, ρ = 999.7 kg/m³ and cp = 4.196 kJ/(kg·K): q = 999.7 × 4.196 × 0.002 × 5 = 41.95 kW (11.9 tons).
Common uses
- Choosing a window or mini-split AC for one room from its floor area
- Checking a cooling coil or an AC in the field from airflow and wet-bulb readings
- Finding the supply airflow for a room’s sensible load, or the temperature rise across a furnace
- Chilled-water and hot-water coil, AHU and fan-coil capacity from flow and ΔT
- HVAC coursework: sensible, latent and total heat with the working shown
Formulas
- Sensible heat of air: q = ρ × cp × V̇ × Δt. For standard air (0.075 lb/ft³, 0.24 Btu/(lb·°F)) that is q = 1.08 × cfm × Δt in Btu/h, or 1.2072 kJ/(m³·K) — 1.2072 W per L/s per K. ASHRAE’s standard-air relation uses 1.10 (1.23 W per L/s per K), which allows for the moisture in typical air; pick either.
- Total heat of air: q_t = 4.5 × cfm × Δh, with the enthalpy h in Btu/lb (1.20 × L/s × Δh in kJ/kg gives W). Latent heat = total − sensible; the sensible heat ratio is sensible ÷ total.
- Water: q = 500 × gpm × Δt in Btu/h — 60 min/h × 8.33 lb/gal × 1 Btu/(lb·°F) — which is 4.18 kW per L/s per K. Other liquids: q = ρ × cp × V̇ × Δt.
- Solved the other way: cfm = q_s ÷ (1.08 × Δt) and Δt = q_s ÷ (1.08 × cfm); gpm = q ÷ (500 × Δt).
- Units: 1 ton of refrigeration = 12,000 Btu/h = 3.517 kW (melting a short ton of ice in a day); 1 MBH = 1,000 Btu/h; 1 kW = 3,412 Btu/h; 1 kcal/h = 1.163 W.
Real air: elevation, temperature and humidity
Air gets thinner with height, so each cfm carries less heat. With Air at your elevation the calculator takes the pressure from the standard atmosphere, p = 101.325 × (1 − 2.25577 × 10⁻⁵ Z)^5.2559 kPa (Z in m), the specific volume v = 0.287042 (t + 273.15)(1 + 1.607858 W) ÷ p at the temperature where the airflow is measured, and cp = 1.006 + 1.86 W kJ/(kg·K). At 5,000 ft the pressure is 12.23 psia, 17 % below sea level. If your airflow reading is already corrected to standard air (scfm), keep the standard factor.
With humidity readings, the humidity ratio W comes from the RH (W = 0.621945 p_w ÷ (p − p_w)) or from the wet bulb (ASHRAE’s psychrometric equation), the saturation pressure from the Hyland–Wexler equations, and the enthalpy from h = 1.006 t + W (2501 + 1.86 t) kJ/kg. The state table shows the dry bulb, wet bulb, dew point, RH, humidity ratio and enthalpy on both sides of the coil.
With standard air the sensible part uses dry air’s specific heat (the 0.24 in 1.08), so latent = total − sensible also holds the small sensible heat of the water vapour and comes out a few percent above the moisture alone, 4,840 × cfm × ΔW (4,840 = 60 × 0.075 × about 1,076 Btu/lb). With Air at your elevation the sensible part uses moist air’s cp, and the latent part is only the moisture condensed.
Room size: the ENERGY STAR chart
ENERGY STAR’s chart for room air conditioners runs from 5,000 Btu/h for 100 up to 150 sq ft to 34,000 Btu/h for 2,000 up to 2,500 sq ft, and its capacities are calculated for an 8-foot ceiling. Its adjustments: reduce the capacity by 10 % if the room is heavily shaded, increase it by 10 % if it is very sunny, add 600 Btu/h for each person beyond two who regularly use the room, and add 4,000 Btu/h if the unit is used in a kitchen. The calculator applies the percentage to the chart value, then adds the people and kitchen allowances, and shows the smallest common window, mini-split and ton size that covers the result.
The chart knows nothing about your climate, insulation, windows or ceiling height. For a sun-baked top-floor room or a hot climate, the AC tonnage calculator’s heat-gain method adds up the walls, roof, glass and people instead. For a whole house or a ducted central system, the standard method is a room-by-room load calculation to ACCA Manual J (8th edition, ANSI/ACCA 2 Manual J – 2016), with the equipment then chosen to ACCA Manual S.
Typical values to check against
- Airflow per ton: a common rule of thumb for comfort cooling is about 400 cfm per ton (roughly 350–450, or 47–60 L/s per kW), the lower end in humid climates for more moisture removal; the calculator warns below 300 or above 500. Check the equipment data for the real figure.
- Coil temperature drop: at 400 cfm per ton and a sensible heat ratio of about 0.75, the air drops roughly 20 °F (11 K) across a cooling coil.
- Chilled water: with the 500 rule the flow per ton is 24 ÷ Δt (°F): 2.4 gpm per ton for a 10 °F rise, 2.0 for 12 °F and 1.5 for 16 °F — a bigger temperature difference needs less water for the same load.
- Hot water: the 500 rule overstates hot water by about 2–3 % (water at 180 °F has a factor of about 487), which “Water at its temperature” corrects.
Sources
- ENERGY STAR — Room Air Conditioners: sizing chart and adjustments
- ASHRAE Handbook—Fundamentals (2017), ch. 1 Psychrometrics: standard atmosphere (eq. 3), saturation pressure (eqs. 5–6), humidity ratio (eqs. 20, 33, 35), specific volume (eq. 26), enthalpy (eq. 30)
- ASHRAE Handbook—Fundamentals, ch. 18 Nonresidential cooling and heating load calculations: standard-air relations q_s = 1.10 Q Δt and q_t = 4.5 Q Δh
- IAPWS-IF97, Industrial formulation for the thermodynamic properties of water and steam: density and specific heat of liquid water
- ACCA Manual J, 8th edition (ANSI/ACCA 2 Manual J – 2016), residential load calculation — referred to, not implemented here
- NIST SP 811 (exact inch, foot, pound and gallon); International Table Btu = 1,055.05585262 J
Limitations
- The room chart is ENERGY STAR’s rule of thumb for one room with an 8-ft ceiling; it ignores climate, insulation, glass and ceiling height, and it does not size a whole house or a central system.
- Standard air (1.08 or 1.10) means sea level and about 70 °F; at altitude use Air at your elevation, unless your airflow is already in standard cfm.
- The air and water formulas assume a steady flow and well-mixed temperatures. Field readings of airflow and temperature are often a few percent off, and that error carries straight into the result.
- Humidity results use the standard-atmosphere pressure for the elevation, not the day’s barometric pressure, which weather shifts by a few percent.
- Water properties are taken at the mean temperature (IAPWS-IF97, liquid water up to 302 °F / 150 °C); for glycol or brine, enter the density and specific heat from the fluid’s data sheet.
Privacy
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Frequently asked questions
How do I calculate BTU from CFM and temperature difference?
Multiply the airflow by the temperature difference and by 1.08: q = 1.08 × cfm × Δt gives Btu/h of sensible heat. 1,200 cfm cooled by 20 °F is 1.08 × 1,200 × 20 = 25,920 Btu/h, about 2.16 tons. The 1.08 is 60 min/h × 0.075 lb/ft³ × 0.24 Btu/(lb·°F) for standard air; in metric, q (kW) = 1.207 × m³/s × Δt (K).
Why is my answer lower than the AC’s rated tons?
Because 1.08 × cfm × Δt is only the sensible heat. A cooling coil also condenses moisture (latent heat), often a fifth to a third of the total. Tick Include humidity and enter the wet-bulb readings: total = 4.5 × cfm × Δh. A coil cooling 1,200 cfm from 80/67 °F to 58/56 °F moves about 41,400 Btu/h in total, of which 28,500 is sensible.
How many cfm per ton?
A common rule of thumb for comfort cooling is about 400 cfm per ton, roughly 350–450 (47–60 L/s per kW); lower airflow dehumidifies more, higher airflow gives more sensible capacity, and the equipment data has the real figure. To work it out for your load, choose Airflow and find the airflow: 3 tons at a sensible heat ratio of 0.75 and a 20 °F drop needs 27,000 ÷ (1.08 × 20) = 1,250 cfm, or 417 cfm per ton.
What is the 500 in 500 × gpm × ΔT?
It is 60 minutes × 8.33 lb per US gallon × 1 Btu/(lb·°F) for water, so Btu/h = 500 × gpm × Δt (°F). 24 gpm warming by 10 °F is 120,000 Btu/h, 10 tons — 2.4 gpm per ton. In metric the same rule is about 4.18 kW per L/s per K. Hot water carries less (about 488 at 170 °F) and glycol less again: choose “Water at its temperature” or “Other fluid”.
What size air conditioner do I need for my room?
For a single room, the ENERGY STAR chart is a quick guide: 150–250 sq ft needs 6,000 Btu/h, 350–400 sq ft 9,000 Btu/h and 450–550 sq ft 12,000 Btu/h, before the adjustments for sun, people and kitchens. In a hot climate or a top-floor room, a heat-gain estimate such as the AC tonnage calculator is more reliable; for a whole house, a Manual J load calculation.
How do I convert tons to BTU or kW?
One ton of refrigeration is 12,000 Btu/h, which is 3.517 kW. So a 1.5-ton AC is 18,000 Btu/h (5.28 kW), and 10 kW is 34,121 Btu/h or 2.84 tons. The Convert tab also gives MBH (thousands of Btu/h) and kcal/h.
Does altitude change the 1.08 factor?
Yes. The factor is proportional to the air density, which falls with height: at 5,280 ft (Denver) and 75 °F it is about 0.88, so the same 1,200 cfm and 20 °F carry about 21,100 Btu/h instead of 25,920. Choose Air at your elevation. If your instrument already reports standard cfm (scfm), keep standard air.
How do I check a furnace’s temperature rise?
Choose Airflow, find the temperature difference, and enter the furnace’s output (not its input) with a sensible heat ratio of 1, and the airflow: 80,000 Btu/h with 1,200 cfm gives 80,000 ÷ (1.08 × 1,200) = 61.7 °F. Rating plates give an allowed rise range; to work out the airflow from a measured rise, find the airflow instead.