Window Heat Loss Calculator
Glass heat loss from U, area, and gap.
What it does
Windows leak heat by conduction through glass. Power equals U value times area times the inside outside gap. A large single pane on a cold night can rival a small heater, while triple glazing cuts the same gap to a fraction.
The tool above runs that product. With 2 m2 at U 2.8 and 20 deg, loss is 112 W with 2.69 kWh a day. The imperial mirror at 21.5 ft2, 0.5, and 36 F gives 387 BTU per h with 9.29 kBTU a day. Conductance (U times A) separates the window quality from the weather gap.
U values anchor the inputs. Single glass sits near 5.8 W per m2K, standard double near 2.8, and high triple near or below 1. Frames leak extra at edges, curtains add a little resistance at night, and sun can briefly reverse the flow by day.
This page models steady glass conduction only. It does not add infiltration, frames, solar gain, or thermostat schedules. Use it to rank glazing choices and to size the gap cost.
How to use
- Pick metric (m2, W) or imperial (ft2, BTU).
- Enter the glass area.
- Enter the U value of the glazing.
- Enter inside minus outside degrees.
- Read heat loss first, then conductance.
- Read daily energy for cost math.
Formula
- A
- Glass area in m2 or ft2.
- U
- Thermal transmittance in W per m2K or BTU per h ft2 F.
- delta_T
- Inside minus outside in C deg (K) or F deg.
Steady state glass conduction with one U, one area, one gap. At zero gap there is no loss. Frames, air leaks, solar gain, and night setback sit outside this line.
Worked example
Example: 2 m2 window at U 2.8
Defaults: metric 2 m2 at U 2.8 with 20 deg gap. Imperial mirror is 21.5 ft2 at 0.5 with 36 F gap.
- Inputs (metric default): 2 m2 glass, U 2.8, gap 20 deg.
- Power: 2.8 times 2 times 20 is 112 W.
- Conductance: 2.8 times 2 is 5.6 W per K.
- Day: 112 times 24 / 1000 is 2.69 kWh per day.
- Imperial mirror (21.5 ft2, 0.5, 36 F) gives 387 BTU per h and 9.29 kBTU per day. Tool shows the same.
Reference table
| Glass and gap | Loss | Day |
|---|---|---|
| 2 m2 U2.8 gap20 | 112 W | 2.69 kWh per day |
| 1.5 m2 U5.8 gap25 | 217.5 W | 5.22 kWh per day |
| 21.5 ft2 U0.5 gap36F | 387 BTU per h | 9.29 kBTU per day |
| 10 ft2 U1.0 gap40F | 400 BTU per h | 9.6 kBTU per day |
| 3 m2 U1.0 gap20 | 60 W | 1.44 kWh per day |
Limits
- Glass only. Frames and leaks excluded.
- Steady state. Thermal mass ignored.
- One U for whole glass. Mixed panes need area weighting.
- No solar gain. Day sun offsets loss.
Common mistakes
- Using R value as U without inverting.
- Mixing m2 with BTU U values.
- Reading conductance as loss without the gap.
- Ignoring edge and frame losses.
- Sizing heat from glass alone for the room.
Try it yourself
Quick checks. Answers sit tucked under each question.
1.5 m2 single glass U 5.8, gap 25?
Loss is 5.8 times 1.5 times 25 is 217.5 W. Conductance is 8.7 W per K. Day is 5.22 kWh per day.
10 ft2 at U 1.0, gap 40 F?
Loss is 1.0 times 10 times 40 is 400 BTU per h. Conductance is 10 BTU per h F. Day is 9.6 kBTU per day.
Questions
How is heat loss figured?
Q = U times A times delta T. Here 2.8 times 2 times 20 is 112 W.
What U values are typical?
Lower U means better glass. Single near 5.8, double near 2.8, triple lower.
What is the temperature gap?
Inside minus outside in the same scale. Here 20 C deg or 36 F deg.
How is daily energy figured?
Multiply watts by hours over 1000. Here 112 times 24 / 1000 is 2.69 kWh per day.
Does this cover the whole window?
No. Frames, seals, curtains, and sun gain sit outside this glass math.
How do I cut the loss?
Yes. Better glass, smaller area, or lower gap all cut loss linearly.