How to use the heat loss calculator
- Enter the room size. Wall area is the four walls minus the window and door openings; the ceiling and floor use the footprint.
- Edit the R-values, U-factors, ACH and ΔT. The form starts with planning examples (R-13 walls, R-38 ceiling, R-19 floor, example window and door U-factors, 0.35 ACH, 70°F), not measured performance.
- Read conduction by assembly plus infiltration, then the total in Btu/h.
Insulation takeoffs stay on the insulation calculator. A planning equipment size from a rule of thumb is on the BTU calculator. Wood stove installation cost, not a heat-load size, is on the wood stove installation cost calculator.
The UA method and the 0.018 factor
Conduction through each surface is Q = A × U × ΔT, with U = 1/R for opaque assemblies. Infiltration is Q = 0.018 × ACH × volume × ΔT. The 0.018 factor is the common 1.08 Btu/h-per-CFM-per-°F conversion divided by 60, because CFM = ACH × volume ÷ 60. Some trade worksheets publish 0.0182 for the same relationship, a slightly higher rounding of air’s volumetric heat capacity; this page uses 0.018.
A worked example: 12×12×8 ft room
A 12 × 12 × 8 ft room with one 15 sq ft window (U-0.35) and one 21 sq ft door (U-0.29), R-13 walls, R-38 ceiling, R-19 floor, 70°F ΔT and 0.35 ACH has about 3,463 Btu/h of conduction and 508 Btu/h of infiltration, or about 3,971 Btu/h total. Those assembly values are the form’s example defaults, not a tested house.
Sources & assumptions
How this calculator gets its numbers. Full method: our methodology.
- Conduction through each surface is Q = A × U × ΔT, with U = 1/R for opaque assemblies. Wall area is the four walls minus the window and door openings entered; ceiling and floor use the room footprint.
- Infiltration is Q = 0.018 × ACH × volume × ΔT. The 0.018 factor is the common 1.08 Btu/h-per-CFM-per-°F conversion divided by 60 (CFM = ACH × volume ÷ 60). Some trade worksheets publish 0.0182 for the same relationship.
- Default R-values (13 / 38 / 19), window and door U-factors (0.35 / 0.29), 0.35 ACH and 70°F ΔT are CalcTheJob editable planning starting points, not measured assembly performance and not a code table.
- This is a UA planning estimate, not an ACCA Manual J load.
Sources
- Michigan Technical Education Center (michigantec.org), Formulas / Conversions (heat-loss worksheet). Publishes Btuh = V × ACH × 0.0182 × ΔT and the equivalent Btuh = CFM × 1.08 × ΔT. This page uses 0.018 (1.08 ÷ 60) and discloses the 0.0182 rounding.
- Michigan Technical Education Center (michigantec.org), Air Transported Heat Loss. Defines 0.0182 Btu/ft³·°F as the volumetric specific heat used in infiltration load calculations, confirming the ACH × volume form of the infiltration term.
Last reviewed . A Manual J or a measured load for the house wins.
Frequently asked questions
How much heat does a 12×12 room lose?
With the form’s example defaults — R-13 walls, R-38 ceiling, R-19 floor, a 15 sq ft U-0.35 window, a 21 sq ft U-0.29 door, 70°F ΔT and 0.35 ACH — about 3,971 Btu/h. Change the R-values and ACH to match the room.
What does the 0.018 infiltration factor mean?
It is 1.08 (Btu/h per CFM per °F) divided by 60, so the infiltration term can be written with ACH and volume instead of CFM. Some worksheets use 0.0182.
Is this a Manual J load?
No. Manual J also accounts for orientation, duct losses, peak load factors and design temperatures for the house. Use this page as a planning UA check.
Are the default R-values from a code table?
No. They are CalcTheJob editable planning starting points. Enter the R- or U-values for the assemblies you actually have.