Heat Transfer Calculator
With this heat transfer calculator, finding the heat rates for different types of heat transfer will be no challenge for you! Heat transfer occurs every time you bring objects of different temperatures into contact with each other. So, there are many examples of heat transfer in daily life when heat is transferred from one object to another.
The concept of heat and the mechanisms of heat transfer are central to engineering and industrial and domestic equipment design. In the text below, we will discuss the heat transfer formula. So let's start by explaining what heat transfer is!
Heat transfer definition – types of heat transfer
Heat transfer occurs when one system comes into contact with another low-temperature system. The energy in the form of heat is transferred from the molecules in the first system to the second system. When the temperature increases, the kinetic energy of the molecules also increases.
There are three different types of heat transfer:
- Conductive heat transfer — the transfer of heat from one molecule to another through direct contact between objects. Conduction is a very effective method of heat transfer in metals, but gases such as air conduct heat poorly.
- Convective heat transfer — usually refers to fluids or gases that exchange heat with other objects during their free movement. See how a fireplace works: heated air expands and rises throughout the room, while cooler air descends towards the fire where it is heated.
- Radiative heat transfer — electromagnetic waves can also transfer heat when they come into contact with matter. The more radiation a body absorbs, the higher the heat transfer. As an example, white objects absorb very little heat — they will be slower to heat up during hot summers.
Heat transfer equations
This heat transfer calculator can match the formula depending on which type of heat transfer you are dealing with. The basic formula for the amount of heat transferred from one object to another (a temperature change of a mass) is as follows:
Q = m · c · ΔT
- Q — Heat transferred;
- m — Mass of a system;
- c — Specific heat, defined as the amount of heat required to increase the temperature of 1 kg of mass by 1 °C; and
- ΔT — Temperature difference, ΔT = T2 − T1.
Note that heat transfer can be either positive (transfer to a system) or negative (transfer from a system).
The heat transfer rate through a slab of material depends on the type of material, the temperature difference ΔT between hot and cold sides, and the path the heat travels. All these quantities are bound by the conduction heat transfer formula:
Q = (k · A · t · ΔT) ÷ l
- Q — Conduction heat transfer;
- k — Thermal conductivity of the material;
- A — Surface area;
- t — Time needed for heat transfer;
- ΔT — Temperature difference, ΔT = Thot − Tcold; and
- l — Thickness of the material.
💡 In developing insulation, look at the l/k ratio in the equation. The lower the conductivity k and the greater the thickness l, the better the insulator.
The other two mechanisms this calculator supports are:
- Convection: Q = h · A · t · ΔT, where h is the convective heat transfer coefficient.
- Radiation (Stefan–Boltzmann law): Q = ε · σ · A · t · (Thot4 − Tcold4), where ε is the emissivity (0–1) and σ = 5.670374×10−8 W/(m²·K⁴) is the Stefan–Boltzmann constant. Temperatures are converted to absolute (K or °R) before being raised to the fourth power.
Unit Systems
This calculator supports two unit systems:
- Metric (SI) — heat in joules (J), conductivity in W/(m·K), area in m², thickness in m, temperature in K or °C, time in seconds. Heat rate is reported in watts (W).
- Imperial / US customary — heat in British Thermal Units (BTU), conductivity in BTU/(hr·ft·°F), area in ft², thickness in ft, temperature in °F, time in hours. Heat rate is reported in BTU/hr.
Every result is also shown in kilowatt-hours (kWh) and kilocalories (kcal) for convenience.
Heat transfer examples in everyday life
- A metal spoon left in a hot cup of tea heats up by conduction.
- A radiator warms a room mostly by convection as warm air circulates.
- You feel the warmth of the Sun or a campfire through radiation.
- Insulation in walls (low k) slows conductive heat loss to keep your home warm.
How to use the heat transfer calculator?
- Choose your unit system — metric (SI) or imperial (US).
- Pick the type of heat transfer: conduction, convection, radiation, or the basic Q = m·c·ΔT.
- Enter the required values. For conduction you can pick a material preset to auto-fill the thermal conductivity.
- Click Calculate. You will get the total heat transferred (Q), the heat transfer rate (power), and the energy in kWh and kcal.
Worked example (conduction)
Heat lost through a 0.1 m thick insulating wall, 10 m², with k = 0.04 W/(m·K) and a 20 K difference over 1 hour (3600 s):
- Q = (0.04 × 10 × 3600 × 20) ÷ 0.1 = 288 000 J ≈ 0.08 kWh
- Heat transfer rate = (0.04 × 10 × 20) ÷ 0.1 = 80 W
FAQs
- What are the three types of heat transfer?
- Conduction (direct molecular contact), convection (movement of a heated fluid or gas), and radiation (electromagnetic waves). This calculator handles all three, plus the basic heat formula Q = m·c·ΔT.
- Can the heat transfer be negative?
- Yes. A negative result means heat is leaving the system (the body is cooling) rather than entering it. This happens when ΔT is negative or when the cold body is hotter than the hot body in radiation.
- What is the difference between heat (Q) and heat transfer rate?
- Heat Q is the total thermal energy transferred over the whole time period (in J or BTU). The heat transfer rate is the power — energy per unit time (in W or BTU/hr) — i.e. Q ÷ t.
- What is a good thermal conductivity for an insulator?
- The lower the better. Fiberglass (≈0.04 W/(m·K)) and styrofoam (≈0.033 W/(m·K)) are excellent insulators, while metals such as copper (≈401 W/(m·K)) conduct heat extremely well.
- What is a BTU?
- One British Thermal Unit is the amount of heat required to raise 1 pound of water by 1 °F. It equals approximately 1055 joules.