- Estimated cooling capacity
- 7,000 BTU/h
- Baseline capacity
- 7,000 BTU/h
How it works
Insulation changes only part of heat loss
Halving wall U-values halves the modeled wall-conduction loss, not the entire heating requirement. Windows, other surfaces and outdoor-air exchange remain. Compare the component totals to see which improvement actually reduces the largest part of your entered load.
Choosing a room estimate or heat-transfer calculation
The BTU calculator separates a room cooling estimate from heat-transfer and steady heating-loss models. Select the method that matches the information you have. US climate presets supply the stated design-temperature reference, while construction and room inputs remain your responsibility. Check whether the result is energy or a rate of heat transfer before comparing it with equipment capacity labels.
Room cooling estimate: formula and assumptions
Choose baseline BTU/h from the floor-area table, then adjust for shade, sun, occupancy, and a kitchen.
The ENERGY STAR table assumes an 8-foot ceiling. Shared area endpoints use the lower capacity row here. Higher ceilings may need more capacity; no unsupported ceiling multiplier is assumed. A complete load calculation also considers insulation, climate, windows and leakage.
Heat energy from temperature change: formula and assumptions
Q = mass × specific heat × temperature change.
Assumes constant specific heat and no phase changes. A negative change removes heat. BTU is energy here, not BTU/h cooling capacity.
US room and building heat transfer: formula and assumptions
Envelope heat transfer = Σ(U × exposed area) × temperature difference. Outdoor-air heat transfer = air heat capacity × air changes per hour × volume × temperature difference / 3600. Add separately established loads.
Use a county reference or your own design temperature, exposed geometry and an assembly reference or actual U-values. Window and door areas are deducted from gross exterior walls. Whole-building mode includes every exterior wall and conditioned floor in its volume. Heating ignores beneficial solar and internal gains. Cooling reports a steady heat-gain subtotal from the entered sources, not a time-dependent design peak or an equipment size. This is not Manual J or an ENERGY STAR certification calculation.
Steady heating loss: formula and assumptions
Heat loss (W) = [UA + air heat capacity × air changes per hour × volume / 3600] × (indoor − outdoor temperature).
Estimate steady sensible heating loss for an entered room or building envelope. Use local design temperatures and measured or specified insulation values. Air heat capacity defaults to 1207 J/(m³·K), an assumption for dry air near sea level and 21 °C. Use operating air changes, not a blower-door ACH50 value. This excludes solar gains, latent heat, ducts, warm-up and equipment sizing; it is not a Manual J calculation.
Worked example
Using the Room cooling estimate example:
- Room area unit
- Square feet
- Room floor area (ft²)
- 300
- Sun exposure
- Typical
- Regular occupants
- 2
- Kitchen
- No
Choose baseline BTU/h from the floor-area table, then adjust for shade, sun, occupancy, and a kitchen.
Example result
Estimated cooling capacity: 7,000 BTU/h
Baseline capacity: 7,000 BTU/h
These are illustrative inputs. Change the values to explore a different scenario.
Formula references
- ENERGY STAR: properly sized room air conditioners (opens in a new tab)
- OpenStax: conduction and thermal resistance (opens in a new tab)
- NREL: building thermal envelope tests, dry-air assumptions (Table A10) (opens in a new tab)
- EPA: county design temperature limits, 2019 edition revised 2020 (opens in a new tab)
- 2015 IECC: assembly U-factor references, Table R402.1.4 (opens in a new tab)
References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.
Common questions
Is this a complete HVAC load calculation?
The ENERGY STAR room mode is an area-based cooling guide. The US design mode uses dated county temperatures, exposed geometry and assembly U-values for steady heating loss or a cooling heat-gain subtotal. Neither is a complete Manual J assessment or equipment selection.
Why do occupancy and kitchen use change the estimate?
Those inputs apply the stated adjustments to the floor-area baseline to account for additional room heat under the guide's assumptions.
What do the US county presets represent?
They use the EPA 2019 Design Temperature Limit Reference Guide, revised in 2020. Its county limits are dated program references, not live weather or guaranteed design conditions at your address. You can enter an established local design temperature instead.
Does selecting a county determine my insulation?
No. Select an assembly reference separately or enter actual whole-assembly U-values. The 2015 code references are comparison assumptions, not measured performance or a statement of current local requirements.
Does the cooling subtotal include humidity and solar heat?
Only the solar, internal, moisture-removal and other gains you enter are included. Zero excludes a contribution. The subtotal does not model thermal storage, a time-dependent peak or equipment capacity.
Prepared by Finflo. See our methodology and editorial policy.