Heatsink Calculator: Junction Temperature and θSA Needed • The Science 360

By Shounak Das · Updated November 2026 · 6 min read

How do you work out the heat sink you need?

The junction temperature is TJ = TA + P × (θJC + θCS + θSA), so the heat sink you need is θSA = (TJ,max − TA) ÷ P − θJC − θCS. A 7805 dropping 12 V to 5 V at 1 A turns 7.04 W into heat; with a 125 °C limit in 25 °C air it needs a heat sink of 8.2 °C/W or better.

For the full 7805 story, read does my 7805 need a heatsink?. To set an LM317's output, see the LM317 formula explained.

What the datasheets give

7805 and LM317 in TO-220: θJC 5 °C/W and θJA 65 °C/W (onsemi MC7800 and LM317 datasheets). The MC7805C draws 3.2 mA quiescent current, typical.
Bare TO-220 limit: (125 − 25) ÷ 65 = 1.54 W in 25 °C air, if you keep to the 125 °C range the electrical tables are specified over.
78L05 in TO-92: θJA 200 °C/W, and no θJC: onsemi says the plastic package sheds its heat through the leads.
IRFZ44N MOSFET: θJC 1.5 °C/W max, θCS 0.50 °C/W greased, θJA 62 °C/W, RDS(on) 17.5 mΩ max, 175 °C maximum junction.
Interface, TO-220 (onsemi AN1040): bare metal, dry 1.2, bare metal, greased 1, mica washer, greased 1.6, mica washer, dry 3.4 °C/W.

Nine heat problems

Each one loads the part, the heat sink (if any) and the thermometer into the calculator.

7805, 12 V to 5 V at 1 A

115 °C

(12 − 5) × 1 + 12 × 0.0032 = 7.04 W. Bare, that would be 482 °C on paper, so a heat sink is not optional. It needs θSA ≤ 8.2 °C/W.

A 6.8 °C/W finned sink with greased mounting gives 115 °C. A 12.5 °C/W channel sink would leave it at 155 °C, over the limit.

Power 7.04 W θSA needed 8.2 °C/W

7805, 9 V in, 0.5 A

157 °C bare

Only 2.03 W, but a bare TO-220 at 65 °C/W reaches 157 °C. Even a 43 °C/W heat sink is enough, so the smallest clip-on fixes it.

Power 2.03 W

7805, 12 V in, 100 mA

73 °C

0.74 W gives 73 °C at the junction: no heat sink needed.

Junction 73 °C

The same 7805 in a 50 °C box, on mica

144 °C

Warmer air and a greased mica washer (1.6 °C/W) push the junction to 144 °C on the 6.8 °C/W sink. It now needs θSA ≤ 4.1 °C/W.

θSA needed 4.1 °C/W

LM317, 12 V to 3.3 V at 1.5 A

13 W

8.7 V × 1.5 A = 13.05 W. It would need θSA ≤ 1.66 °C/W, and even a 4.4 °C/W sink leaves the junction at 161 °C. This is a job for a switching regulator.

θSA needed 1.66 °C/W

IRFZ44N switching 10 A

2.8 W

10² × 17.5 mΩ × 1.6 = 2.8 W, using the datasheet's normalized curve (about 1.6 times at 100 °C). Bare it reaches 199 °C, past 175 °C; a 24.4 °C/W clip brings it to 99 °C.

Bare 199 °C

IRFZ44N at 20 A

11.2 W

Twice the current is four times the heat: 11.2 W. θSA must be 11.4 °C/W or less; a 6.8 °C/W sink gives 124 °C.

θSA needed 11.4 °C/W

78L05 in TO-92

72 °C

9 V to 5 V at 50 mA is 0.23 W, about 72 °C. At 12 V and 100 mA it would be 0.75 W and 174 °C: the TO-92 tops out near 0.5 W in 25 °C air.

Bare limit 0.5 W

The Aavid catalogue example

10.7 °C/W

6 W, θJC 3 °C/W, 0.5 °C/W interface, 150 °C limit and 65 °C air: (150 − 65) ÷ 6 − 3 − 0.5 = 10.7 °C/W, the same answer the catalogue gives.

θSA 10.7 °C/W

7805 power and heat sink needed (25 °C air, 125 °C limit, greased)

Input	0.1 A	0.5 A	1 A
7 V	0.22 W, none	1.02 W, none	2.02 W, ≤ 43.4
9 V	0.43 W, none	2.03 W, ≤ 43.3	4.03 W, ≤ 18.8
12 V	0.74 W, none	3.54 W, ≤ 22.3	7.04 W, ≤ 8.2
15 V	1.05 W, none	5.05 W, ≤ 13.8	10.05 W, ≤ 4
24 V	1.98 W, ≤ 44.6	9.58 W, ≤ 4.4	19.1 W, too much

Power in watts, then the heat sink thermal resistance needed in °C/W; "none" means the bare TO-220 stays under 125 °C.

How the calculator works

Power: (Vin − Vout) × I + Vin × Iq for a linear regulator, I² × RDS(on) × a hot factor for a MOSFET, or the watts you type.

Bare: TJ = TA + P × θJA. On a heat sink: TJ = TA + P × (θJC + θCS + θSA), with the case and heat sink temperatures in between.

The heat sink needed is the θSA that puts TJ exactly at your limit; the calculator suggests the first catalogue sink at or below it.

What the model leaves out

Airflow and orientation. Catalogue θSA values are for natural convection; a fan lowers them, an enclosure raises them.
Board copper. θJA depends on the board; surface-mount parts lean on copper area, which this does not model.
Transients. Short pulses heat the part less than the steady-state figure suggests; this is the steady state.

Common misconceptions

"A hot case means a failing part." In the 1 A example the tab sits at 80 °C while the junction, at 115 °C, is still inside its 125 °C limit.

"A small load needs no heat sink." It is the voltage dropped times the current that counts: a 7805 from 9 V at 0.5 A already passes 125 °C bare.

"A bigger heat sink always fixes it." θJC and the interface set a floor; at high power no heat sink is enough.

Frequently asked questions

How do I calculate the heat sink I need?

θSA = (TJ,max − TA) ÷ P − θJC − θCS. Pick a heat sink with a thermal resistance at or below that number.

How much power can a TO-220 handle without a heat sink?

With θJA 65 °C/W, about 1.5 W in 25 °C air for a 125 °C junction.

Does a 7805 need a heat sink?

Above about 1.5 W, yes. That is roughly (Vin − 5) × I; 12 V in at 1 A is 7 W and needs about 8 °C/W.

What is θCS?

The thermal resistance between the part's tab and the heat sink. onsemi measures about 1.0 °C/W greased and 1.6 °C/W with a greased mica washer for TO-220.

Why does a MOSFET get hotter than its RDS(on) suggests?

RDS(on) rises with temperature. The IRFZ44N datasheet curve shows about 1.6 times the 25 °C value at 100 °C.