Overall heat transfer coefficient calculator
Get the U-value of a wall from its layers and air films: 0.2 m of brick with films of 10 and 25 W/(m²·K) comes to just under two and a half W/(m²·K). Each resistance is listed, so you see which layer limits heat loss. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- H inside
- 10
- H outside
- 25
- Layer count
- 1
- R inside
- 0.1
- R outside
- 0.04
- R conduction
- 0.2777777778
The overall heat transfer coefficient of a plane wall, written U, is the single conductance that turns an air-to-air temperature difference into heat flux through the whole assembly. It is not a material property: it combines the inside convective film, every solid layer's conduction resistance, and the outside convective film.
What overall U means for a plane wall assembly
In one-dimensional steady heat transfer the driving temperature difference is the indoor air temperature minus the outdoor air temperature, not the two solid surface temperatures. Between those two air streams heat must cross an inside convective film, then every solid layer in series, then an outside convective film. Each of those steps is a thermal resistance.
Each solid layer contributes its conduction resistance L over k, where L is thickness in metres and k is thermal conductivity in watts per metre-kelvin. Because the layers sit one behind another, the resistances add. The overall heat transfer coefficient U is defined as the reciprocal of that total resistance, so the heat flux in watts per square metre is simply q equals U times Delta T.
The result is a plane-wall series U in watts per square metre-kelvin, with the intermediate resistances exposed so a student can mark which term dominates and a specifier can see whether another centimetre of insulation is worth adding.
When to use this U-value calculator, and when not
Heat-transfer instructors need a deterministic oracle that reproduces the plane-wall overall-U identity so lab write-ups can compare a hand calculation with a machine result without spreadsheet rounding drift, and so a zero film coefficient is marked wrong instead of quietly returning infinity. Building-physics notebooks and HVAC takeoff scripts use the same stack when they already have ISO or ASHRAE surface resistances and a list of layer thicknesses and conductivities and want U, not an imperial R-per-inch batt calculator. Failed validation does not consume balance.
This landing is not the insulation R-value calculator, which multiplies thickness in inches by R per inch and reports an IP U-factor. It is not the heat-transfer-coefficient unit converter, which only changes W/(m²·K) into other film units. It is not the Fourier conduction rate Q equals k A Delta T over L for a single slab with known surface temperatures.
Keep units SI and consistent, stay inside the published magnitude bounds, and you get a reproducible plane-wall U for homework, LMS plugins, and edge workers without standing up a full energy-model engine of your own.
What you can do with it
Envelope takeoff from known films and layers
Turn ISO or ASHRAE surface resistances plus a list of thicknesses and conductivities into a plane-wall U-value for a wall, roof, or partition sketch.
FAQ
Why are both inside and outside film coefficients required?
Overall U is an air-to-air conductance. Omitting a film would describe surface-to-surface conduction only.
How is this different from an insulation R-value calculator?
This tool builds SI series resistances from 1/h films and L/k layers. An insulation R-value calculator multiplies inches by R-per-inch and reports an IP U-factor; that is a different search and a different contract.
How do I calculate the U-value of a wall?
Add one over the inside film coefficient, each layer's thickness divided by its conductivity, and one over the outside film coefficient, then take the reciprocal. In the brick example, the sum is 0.1 plus 0.2 divided by 0.72 plus 0.04.
How can I tell which layer matters most?
Look at the resistances listed for each step. In the brick example the films contribute 0.1 and 0.04, while the brick itself contributes most of the total, so a thicker or better insulated layer would change U the most.
Can it give the heat loss through the wall?
Yes. Enter the air-to-air temperature difference to get the heat flux per square metre, and add the wall area to get the total heat rate. The sign of the result shows which way the heat flows.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/eng/overall-u \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"h_inside":10,"h_outside":25,"layers":[{"name":"brick","thickness":0.2,"conductivity":0.72}]}'const res = await fetch("https://api.kit.forhosting.com/eng/overall-u", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"h_inside": 10,
"h_outside": 25,
"layers": [
{
"name": "brick",
"thickness": 0.2,
"conductivity": 0.72
}
]
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/eng/overall-u",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"h_inside": 10,
"h_outside": 25,
"layers": [
{
"name": "brick",
"thickness": 0.2,
"conductivity": 0.72
}
]
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/eng/overall-u", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"h_inside":10,"h_outside":25,"layers":[{"name":"brick","thickness":0.2,"conductivity":0.72}]}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"h_inside":10,"h_outside":25,"layers":[{"name":"brick","thickness":0.2,"conductivity":0.72}]}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/eng/overall-u", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"h_inside": 10,
"h_outside": 25,
"layers": [
{
"name": "brick",
"thickness": 0.2,
"conductivity": 0.72
}
]
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "eng.overall_u",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_layers | 32 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
This overall heat transfer coefficient calculator accepts the two film coefficients and an ordered list of thicknesses and thermal conductivities, forms the series resistances R_si equals one over h_i, R_j equals L_j over k_j, and R_se equals one over h_o, and returns U equals one over R_total together with each resistance so you can see which path dominates. Zero or negative film coefficients or layer properties fail with invalid_input instead of a silent infinity.
What overall U means for a plane wall assembly
The inside film resistance is one over the inside heat-transfer coefficient h_i. The outside film resistance is one over h_o. Searchers who type overall heat transfer coefficient calculator, plane wall U-value, or wall assembly U from convection and conduction already have h_i, h_o, and a stack of L and k and want that single SI number, not an insulation R-per-inch takeoff and not a unit converter. This page answers that textbook identity. It does not invent still-air film tables, wind speeds, or emissivities: you supply the two film coefficients that belong to your problem, exactly as Incropera-style homework and ISO-style envelope checks do once the surface resistances have been chosen.
Input contract, series resistances, and the returned U-value
At most thirty-two layers are accepted. Optional output keys are omitted entirely when you did not send the matching input, never returned as null. POST a JSON object with three required ingredients. h_inside is the inside film coefficient in watts per square metre-kelvin; aliases h_i, hi, h_inner, inside_h, and hin are accepted. h_outside is the outside film coefficient in the same units; aliases h_o, ho, h_outer, outside_h, and hout are accepted. layers is a non-empty array of solid layers, indoor face first, each with thickness in metres and conductivity in watts per metre-kelvin; optional name, material, or label is kept for display. Array aliases assembly, wall_layers, and materials work the same way. Optional delta_t is the air-to-air temperature difference in kelvin; when it is present the response includes heat_flux equal to U times Delta T. Optional area in square metres, together with delta_t, adds heat_rate equal to U times A times Delta T. Every required magnitude must be a finite number strictly greater than zero; missing fields, empty strings, non-numeric text, zero films, zero thickness, and zero conductivity fail with invalid_input naming the field, and those failures are not billed. On success the body echoes the rounded film coefficients, reports layer_count and a normalized layers array with each L, k, and L/k, and sets r_inside, r_outside, r_conduction, and r_total in square metre-kelvin per watt. The fields u_value and overall_u both carry one over R_total so clients can pin either name. formula restates U equals one over the sum of one over h_inside, the sum of L over k, and one over h_outside; method is always series_resistance_plane_wall.
Where this U-value API earns its keep, and what it is not
Envelope-compliance checkers can pin u_value in tests after they change a layer thickness.
Integration use cases
Heat-transfer homework and lab checks
Give students h_i, h_o, and a brick or multilayer stack and verify U = 1 / (1/h_i + Σ L/k + 1/h_o) against the textbook series-resistance method.
Scripts and LMS auto-grading
Pin u_value and the resistance breakdown in tests so a zero film or a missing layer fails with invalid_input instead of an invented conductance.
Technical questions
How do I calculate the overall heat transfer coefficient of a wall?
Send positive h_inside and h_outside in W/(m²·K) and a layers array of {thickness, conductivity} in metres and W/(m·K). The API returns U = 1 / (1/h_i + Σ(L/k) + 1/h_o).
What units does the overall U-value use?
SI throughout: films in W/(m²·K), thickness in metres, conductivity in W/(m·K), resistances in m²·K/W, and u_value in W/(m²·K). Keep every input in that coherent set.
What does the overall heat transfer coefficient calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.