Calculate heat capacity (Q=mcΔT), heat transfer rate, heat index (feels-like temperature), watts to BTU/heat conversion, specific heat, water heating energy, and sensible heat — all 6 modes in one tool.
Enter specific heat valueEnter a valid specific heat.
In °C or K (same value)Enter a valid temperature change.
Material of conducting bodySelect a material.
Enter conductivityEnter a valid value.
Area of the conducting surfaceEnter a valid area.
Hot side minus cold side (Kelvin)Enter a valid temperature difference.
Material thickness in metersEnter a valid thickness.
Duration of heat transferEnter a valid time.
Select your temperature unitSelect a unit.
Minimum 80°F / 27°C for heat indexEnter a valid temperature.
Minimum 40% for heat index calculationEnter humidity between 0 and 100.
Electrical power in wattsEnter a valid wattage.
How long the device runsEnter a valid duration.
Enter volume (liters or gallons)Enter a valid volume.
Liters or US gallonsSelect a unit.
Starting water temperatureEnter a valid temperature.
Desired final temperatureEnter a valid temperature above initial.
Your water heater wattageEnter a valid wattage.
Typical electric = 95-99%Enter efficiency 1-100.
Mass of the substanceEnter a valid mass.
Temperature rise or dropEnter a valid temperature change.
Known heat energy in kilojoulesEnter a valid energy value.
Result
--
⚠️ Disclaimer: Results are based on standard physics formulas for educational and engineering estimation purposes. Always consult a qualified engineer for safety-critical thermal design applications.
Was this calculator helpful?
✓ Thanks for your feedback!
📚 Sources & Methodology
All heat calculations on this page use verified formulas from the following authoritative sources:
NIST Standard Reference Database 69 — Specific heat capacity and thermodynamic properties of pure substances
ASHRAE Handbook of Fundamentals 2021 — Heat transfer coefficients, thermal conductivity values, and HVAC calculations
NWS Rothfusz Regression (1990) — Official National Weather Service heat index formula used by NOAA
Incropera & DeWitt, Fundamentals of Heat and Mass Transfer, 8th Ed. — Conduction, convection, and radiation heat transfer equations
Complete Guide to Heat Calculations
Heat Capacity Calculator — Q = mcΔT Explained
The heat capacity formula Q = mcΔT is the most fundamental calculation in thermodynamics. It answers a simple question: how much energy does it take to change the temperature of a substance? Q is thermal energy in Joules, m is mass in kilograms, c is the specific heat capacity in J/(kg·K), and ΔT is the temperature change in Kelvin or Celsius (the scale difference doesn't matter for changes).
This formula applies to sensible heat — heat that causes a measurable temperature change. It is used in HVAC system design, industrial process heating, water heater sizing, electrical component thermal management, and any engineering problem involving heating or cooling a material.
Heat Capacity Formula
Q = m × c × ΔTQ = heat energy (Joules) | m = mass (kg) | c = specific heat (J/kg·K) | ΔT = temp change (K or °C)Example: Heat 5 kg of water by 80°C = 5 × 4,186 × 80 = 1,674,400 J = 1,674 kJ = 465 Wh
Heat Transfer Calculator — Conduction Formula
Heat transfer by conduction follows Fourier's Law: the rate of heat flow (in watts) through a solid equals the thermal conductivity multiplied by the cross-sectional area and temperature gradient. This is essential for designing heat sinks for electronics, insulating buildings, and sizing transformers. A copper heat sink (k=385 W/m·K) conducts heat 15,000 times faster than fiberglass insulation (k=0.04 W/m·K).
Fourier's Law of Heat Conduction
Q/t = k × A × ΔT / d (heat transfer rate in Watts)Q = k × A × ΔT × t / d (total heat energy in Joules)k = thermal conductivity (W/m·K) | A = area (m²) | d = thickness (m)Thermal Resistance: R = d / (k × A) in K/W
Heat Index Calculator — What Is Feels-Like Temperature?
The heat index (also called apparent temperature or feels-like temperature) combines air temperature and relative humidity to describe how hot it actually feels to the human body. High humidity prevents sweat from evaporating, reducing the body's ability to cool itself. At 95°F (35°C) with 65% humidity, the heat index reaches approximately 113°F (45°C) — dangerously hot. The NOAA heat index formula is only valid above 80°F (27°C) and 40% relative humidity.
⚠️
Heat danger thresholds: Heat Index 91–103°F = Caution (fatigue possible). 103–124°F = Danger (heat cramps/exhaustion likely). Above 124°F = Extreme Danger (heat stroke imminent). These thresholds are from the National Weather Service.
Watts to Heat (BTU) Conversion
Electrical power dissipated as heat follows strict conversion constants. Every watt of electricity that is not converted to useful work becomes heat. This is critical for data center cooling, electrical enclosure design, motor sizing, and HVAC calculations. The standard conversion: 1 Watt = 3.41214 BTU/hour.
Calculating the energy to heat water uses the standard heat capacity formula with water's specific heat of 4,186 J/(kg·K). Since water has a density of 1 kg/liter, mass in kg equals volume in liters. Electric water heaters are typically 95–99% efficient, meaning almost all electrical energy becomes heat. Gas heaters are 60–80% efficient. Heat pump water heaters achieve 200–300% efficiency (COP) by moving heat rather than generating it.
Specific Heat Capacity Reference Table
Material
Specific Heat c (J/kg·K)
Application
Water (liquid)
4,186
Boilers, cooling systems, HVAC
Ice
2,090
Cold storage, refrigeration
Steam (100°C)
2,010
Steam heating systems
Air (dry)
1,005
HVAC airflow, forced air cooling
Transformer oil
1,900
Electrical transformer cooling
Concrete
880
Thermal mass, slab heating
Aluminum
897
Heat sinks, busbars
Iron / Steel
490
Motor cores, structural steel
Copper
385
Conductors, heat exchangers
Glass
840
Windows, thermal glazing
Granite
790
Countertops, thermal storage
Wood (oak)
1,700
Building construction
Heat Transfer vs Sensible Heat vs Latent Heat
Sensible heat changes temperature without phase change — calculated with Q = mcΔT. Latent heat changes phase without temperature change: water’s latent heat of vaporization is 2,260 kJ/kg at 100°C, and latent heat of fusion is 334 kJ/kg at 0°C. Heat transfer describes the rate of heat movement (watts) through a medium by conduction, convection, or radiation. Understanding which type applies to your problem is the first step in any thermal calculation.
Newton’s Law of Cooling — How Objects Cool Over Time
When a hot object cools in a cooler environment, the cooling rate is proportional to the temperature difference: dT/dt = −k(T − T_ambient). The solution T(t) = T_ambient + (T_initial − T_ambient) × e^(−kt) describes exponential cooling. This is used in electronics thermal management to predict how quickly a component cools after power is removed, and in food safety to determine how long food stays at safe temperatures.
💡
Engineering rule of thumb: For HVAC heat load calculations, use 250–400 BTU/hr per person for body heat, 3.41 BTU/hr per watt of lighting, and 3.41 BTU/hr per watt of equipment. Add 10–15% safety margin for equipment sizing.
❓ Frequently Asked Questions
Q = m x c x delta-T is the fundamental heat capacity formula. Q is thermal energy in Joules, m is mass in kg, c is specific heat capacity in J/(kg*K), and delta-T is the temperature change. For water (c = 4,186), heating 2 kg by 50 degrees C requires Q = 2 x 4,186 x 50 = 418,600 J = 418.6 kJ = 116 watt-hours of energy.
Heat transfer by conduction: Rate = k x A x delta-T / d (watts). k is thermal conductivity (W/m*K), A is cross-sectional area (m2), delta-T is temperature difference across the material, d is thickness in meters. For convection: Rate = h x A x delta-T, where h is the convective heat transfer coefficient (W/m2*K). The total thermal resistance is R = d/(k*A) in K/W.
The heat index is the apparent or feels-like temperature accounting for humidity. High humidity prevents sweat evaporation, making temperatures feel hotter. The NOAA Rothfusz equation calculates it from temperature (F) and relative humidity (%). It is only valid above 80 degrees F and 40% humidity. At 95 F / 65% RH, the heat index is approximately 113 F. Use our calculator above for exact values.
1 watt = 3.41214 BTU per hour. Multiply watts by 3.41214 to get BTU/hr. A 2,000-watt electric heater produces 2,000 x 3.41214 = 6,824 BTU/hr. 1 ton of air conditioning = 12,000 BTU/hr = 3,517 watts. For electrical enclosure heat dissipation, sum all component wattages and multiply by 3.41214 to find total BTU/hr that must be removed.
Specific heat is the energy in Joules needed to raise 1 kg of a material by 1 Kelvin. Water = 4,186 J/kg*K (extremely high, ideal for cooling), aluminum = 897, steel = 490, copper = 385, air = 1,005, transformer oil = 1,900. High specific heat means the material can absorb large amounts of heat with small temperature rise, which is why water is used in cooling systems and radiators.
Energy = volume (liters) x 4,186 x temperature change (Joules, since 1 liter water = 1 kg). To heat 100 liters from 15 to 60 degrees C: Q = 100 x 4,186 x 45 = 18,837,000 J = 18,837 kJ = 5.23 kWh. At $0.12/kWh that costs $0.63. A 3,000-watt heater at 95% efficiency takes 5,230/0.95/3,000 = 1.84 hours. Use the Water Heating tab above for any scenario.
Sensible heat causes a measurable temperature change (Q = mcΔT). Latent heat changes the phase of a substance (solid to liquid, liquid to gas) with no temperature change. Water's latent heat of vaporization = 2,260 kJ/kg at 100 degrees C. Water's latent heat of fusion = 334 kJ/kg at 0 degrees C. In HVAC, the Sensible Heat Ratio (SHR) describes what fraction of total heat removal is sensible vs latent (dehumidification).
Heat loss = A x delta-T / R-value, where A is wall area in m2, delta-T is inside-outside temperature difference in Kelvin, and R-value is thermal resistance in m2*K/W. In US customary units, use ft2*F*hr/BTU. R-13 fiberglass batts (R = 2.29 m2*K/W), a 20 m2 wall, 15 degrees K difference: heat loss = 20 x 15 / 2.29 = 131 watts. Double-pane windows have R = 0.35 m2*K/W, losing 857 watts in the same conditions.
In electrical systems, all power that isn't converted to useful work becomes heat. Resistive losses follow P = I^2 x R. A motor with 90% efficiency running at 10 kW input dissipates 10,000 x (1 - 0.90) = 1,000 watts as heat = 3,412 BTU/hr. Electrical enclosure thermal management must remove this heat to prevent overheating. Rule of thumb: add 10-15% margin above calculated heat load when sizing cooling equipment.
Newton's Law of Cooling: dT/dt = -k(T - T_ambient), meaning the cooling rate is proportional to the temperature difference. Solution: T(t) = T_ambient + (T_initial - T_ambient) x e^(-kt). The constant k depends on the object's surface area, thermal conductivity, and convective coefficient. Used in electronics to predict transient thermal response and in food safety to determine cooling time to safe storage temperatures.
Specific heat (J/kg*K) measures heat storage capacity. Thermal conductivity (W/m*K) measures heat flow rate through a material. Copper: high conductivity (385 W/m*K) but moderate specific heat (385 J/kg*K) = ideal for heat sinks that quickly dissipate heat. Water: low conductivity (0.6 W/m*K) but very high specific heat (4,186 J/kg*K) = ideal for thermal storage but poor conductor. Insulation materials have low conductivity to slow heat flow.
Basic HVAC rule: 20 BTU per square foot for moderate climates, 25-30 BTU/sqft for hot/sunny rooms. Add: 600 BTU/hr per person regularly in the room, 4,000 BTU/hr if adjacent to kitchen, 1,000 BTU/hr extra for very sunny rooms. A 300 sqft bedroom needs approximately 8,000 BTU/hr (0.67 tons). Convert to watts: 8,000 / 3.41214 = 2,344 watts cooling capacity. Always size up rather than down for reliable comfort.