Heat Transfer Calculator
Conduction through a wall in one card: the leak rate from k, area, temperature difference and thickness, with the R-value cross-check on the second road and a day's loss priced in kilowatt-hours.
Heat Transfer Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Steady-state conduction only — no storage, no transients
- Single layer; real walls are composites with studs
In short: A fibreglass wall (≈ 0.04 W/m·K, approximate table value), 12 m² of it, 100 mm thick, 25 K between the sides: rate = k·A·ΔT/L = 0.04 × 12 × 25 / 0.1 = 120.000000 W. The R-value card walks the same number by the second road: R = L/k = 2.500000 m²K/W, U = 0.400000, and U·A·ΔT = 120.000000 W. Double the insulation and the leak halves — length is bought linearly. A day of this leak is 10,368,000 J = 2.880000 kWh of pure loss, which is why the card sits next to your heating bill, not your physics text.
Formula
rate = k·A·ΔT / L · R = L/k, U = 1/R · same number, second road: U·A·ΔT
Fourier's conduction law is a walk with a toll: heat crosses a thickness L at a rate set by the material's conductivity k, the area A, and the temperature difference driving it. The R-value is the same law turned into a shopping number — thickness per conductivity — and the page computes both roads so you can watch them agree.
Worked Example
- Enter conductivity, area, thickness and the temperature difference.
- Read the leak rate in watts.
- Cross-check the R-value card — the same watts by the builder's road.
- Read a day's loss in kWh before quoting insulation paybacks.
Defaults: 120.000000 W; R = 2.500000 m²K/W; a day = 10,368,000 J = 2.880000 kWh. Double the thickness: 60.000000 W.
Strengths & Limits Of This Model
Where this engine is strong
- R-value road computed to the same watts
- A day's leak priced in kWh
Where it stops
- No radiation or convection films
- One homogeneous layer, no framing fraction
Practical Use Cases
Insulation
compare thicknesses before buying
Thermal design
heatsink and enclosure leak rates
Teaching
Fourier's law with a bill attached
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Heat Transfer Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does thickness help linearly?
Because the law divides by L: doubling the path halves the rate, every time, at the same ΔT. That linearity is why insulation is sold in thicknesses and why the first 100 mm does as much work as the next 100 — diminishing returns arrive through COST, not through the physics.
Why does copper wreck the insulation chart?
Conductivity spans four orders of magnitude across the material chips: copper ≈ 385 against fibreglass ≈ 0.04 — ten thousand times. That is why cooking pans are metal and jumpers are not. The chips compute your wall in any of them so the gulf is a number you watched appear.
Is R-value just marketing for L/k?
It is L/k dressed for the hardware store: thickness over conductivity, per square metre. The page computes the R road and the raw road to the SAME watts, which is the whole point — whatever the packaging, the law is one line of arithmetic.
What does ΔT = 0 mean physically?
Thermal equilibrium: both sides at one temperature, nothing flows, honestly zero. The page prints the zero because 'no difference, no leak' is a real state — it is the definition of the driving term, not a failure of the calculator.
Why must ΔT be warm minus cold?
Because the page prices a leak OUT; a negative difference would be a leak inward with the direction hidden in a sign. Enter the magnitude as warm minus cold and swap the words 'winter' and 'summer' yourself — the watts do not care which side is which, only how far apart they are.
Does the area matter more than the thickness?
They enter symmetrically — multiply and divide — but budgets rarely treat them alike: area is fixed by the house, thickness by the wallet. The double-path card exists because thickness is the lever you can actually pull after the walls exist.
Where is radiation in all this?
Out of scope. Radiant transfer grows with the FOURTH power of absolute temperature and crosses vacuum, which is a different law (and why infrared matters in lofts and thermos flasks). This page prices conduction only, and says so rather than smuggling terms.
How does this pair with the specific-heat page?
As rate versus bill: this page prices joules PER SECOND through a wall; that page prices the joules needed to warm a mass. Multiply the leak by a day and you have exactly the energy that page turns into temperature climbs.