Heater resistance calculator: ohms, watts and volts for cartridges and beds

The formula

A heater cartridge or DC heated bed is a resistor, so it should read its rated voltage squared divided by its rated watts. A 24 V 40 W cartridge reads 14.4 Ω; a 12 V 40 W one reads 3.6 Ω. HyperPhysics gives the three forms of Ohm’s law for power in a resistor:

R = V² / P     resistance in ohms, from rated volts and watts
I = V / R      current in amps
P = V² / R     power in watts at any voltage, from a resistance

Power follows the square of the voltage. Double the voltage across the same heater and it makes 4 times the power. Halve it and it makes a quarter.

The meter’s own test leads add resistance to every reading. Fluke puts it at 0.2 to 0.5 Ω. The calculator subtracts whatever your meter shows with the probes touching. On a 3.6 Ω cartridge, 0.3 Ω of lead resistance is an 8% error.

Where the defaults come from

Input	Default	Source
Rating	24 V, 40 W	E3D’s standard V6 cartridge; Prusa’s hotend heater check uses the same range
Reading	14.4 Ω	E3D: initial resistance of its 24 V 40 W Revo HeaterCore at 23 °C
Probes touching	0 Ω	Assumes the meter’s REL or zero mode is on
Tolerance	±10%	Watlow FIREROD cartridge heaters: resistance +10%, −5%

Watlow’s figure covers its cartridges larger than 1/8 in. (3.2 mm), which includes the 6 mm cartridges in most hotends. Where E3D or Prusa publish a range for a rating, the calculator uses theirs instead of the tolerance.

Worked example with the defaults

What should a 24 V 40 W cartridge read?

R = 24² / 40  = 576 / 40 = 14.4 Ω
I = 24 / 14.4 = 1.67 A
±10%          = 12.96 to 15.84 Ω
E3D's range   = 12.3 to 15.1 Ω
on 12 V:  P = 12² / 14.4 = 10 W

What is a cartridge that reads 3.9 Ω? Touch the probes together first. Say they read 0.3 Ω:

R = 3.9 − 0.3 = 3.6 Ω
on 12 V:  P = 12² / 3.6 = 40 W, I = 3.33 A
on 24 V:  P = 24² / 3.6 = 160 W, I = 6.67 A

3.6 Ω falls inside E3D’s 3.1 to 3.8 Ω range for a 12 V 40 W cartridge. It belongs on a 12 V printer.

Common heater ratings and their resistance

Resistance by rated voltage and power. Nominal values are V² / P. Published ranges are measured cold, as the maker lists them.

Hotend heater cartridges

Rating	Nominal	Current	Published range
12 V 30 W	4.80 Ω	2.50 A	4.2 to 5.7 Ω (E3D)
12 V 40 W	3.60 Ω	3.33 A	3.1 to 3.8 Ω (E3D)
12 V 50 W (Slice Engineering)	2.88 Ω	4.17 A	none published
24 V 30 W	19.2 Ω	1.25 A	16.7 to 22.6 Ω (E3D)
24 V 40 W	14.4 Ω	1.67 A	12.3 to 15.1 Ω (E3D, Prusa)
24 V 50 W (Slice Engineering)	11.5 Ω	2.08 A	none published
24 V 60 W (E3D Revo HeaterCore)	9.6 Ω	2.50 A	9.5 Ω at 23 °C (E3D)

E3D’s Revo HeaterCore is a PTC (positive temperature coefficient) heater: its “power reduces as it gets hotter.” Its cold reading still follows V² / P, which is why E3D lists 14.4 Ω for the 24 V 40 W core.

DC heated beds

Bed	Rating	Resistance	Current at 24 V
Prusa MK3, MK3.9, MK4 and MK4S, XL, CORE One	24 V	2.0 to 4.0 Ω (Prusa)	6 to 12 A
Prusa MK52 replacement bed	24 V, 6.25 A (150 W derived)	3.84 Ω derived	6.25 A
Prusa MINI	24 V	4.5 to 6.5 Ω (Prusa)	3.7 to 5.3 A
Creality Ender 3, Ender 3 Pro, Ender 3 V2	24 V, 220 W	2.62 Ω derived	9.17 A
Creality Ender 3 S1	24 V, 270 W	2.13 Ω derived	11.25 A
Creality Ender 3 Max	24 V, 280 W	2.06 Ω derived	11.67 A
Prusa MK2 and MK2S	12 V	0.9 to 1.1 Ω (Prusa)	11 to 13 A at 12 V

“Derived” means the listing gives volts and watts or amps, and the resistance comes from R = V² / P or R = V / I.

What resistance should a heater cartridge have?

A heater cartridge should read its rated voltage squared divided by its rated watts. A 24 V 40 W cartridge reads 14.4 Ω and a 12 V 40 W one reads 3.6 Ω. E3D accepts 12.3 to 15.1 Ω and 3.1 to 3.8 Ω for those two, measured cold with the cartridge unplugged from the board.

E3D’s instruction for a reading outside its range is plain: “do not attempt to heat up the heater cartridge.” A reading well below the range points to a 12 V part or a fault. If the meter shows OL, as it does with the probes apart, the heater or a lead is broken.

How do you tell a 12V heater cartridge from a 24V one?

Measure its resistance cold. At the same wattage, a 24 V cartridge reads 4 times the resistance of a 12 V one. A 40 W cartridge reads about 14.4 Ω if it is built for 24 V and about 3.6 Ω for 12 V. A 30 to 50 W cartridge reading under 6 Ω is a 12 V part.

In the table above, 12 V cartridges from 30 to 50 W sit between 2.88 Ω (nominal) and 5.7 Ω. The 24 V ones sit between 11.5 Ω (nominal) and 22.6 Ω. The gap is wide enough for a meter to settle the question without the label. Switch the calculator to “A meter reading” and pick “Hotend heater cartridge” to see the match.

What happens if you put a 12V heater cartridge on 24V?

It makes 4 times its rated power, because power rises with the square of the voltage. A 12 V 40 W cartridge at 3.6 Ω would put out 160 W on 24 V and draw 6.67 A, twice its rated current. Across E3D’s 3.1 to 3.8 Ω range, that is 152 to 186 W.

The wiring and the board’s heater output carry 4 times the 1.67 A a 24 V 40 W cartridge draws. Fit a 24 V cartridge of the same wattage instead.

The opposite mistake underpowers the heater. A 24 V cartridge on 12 V makes a quarter of its power: a 24 V 40 W cartridge gives 10 W and draws 0.83 A, so it heats far more slowly. To size the supply for a new heater, use the power supply calculator.

What resistance should a 3D printer heated bed have?

A 24 V DC heated bed of 150 to 280 W reads about 2 to 4 Ω. Prusa lists 2.0 to 4.0 Ω for its MK3 to CORE One beds and 4.5 to 6.5 Ω for the MINI. A 220 W Ender 3 bed works out to 2.62 Ω. Prusa’s 12 V MK2 bed reads 0.9 to 1.1 Ω.

Bed resistance is low enough that lead resistance matters. On a 2.62 Ω bed, 0.3 Ω of leads reads as 2.92 Ω, which the formula turns into 197 W instead of 220 W. Zero the probes before trusting the number.

Mains (AC) beds

This calculator is for 12 V and 24 V DC heaters. A mains bed has much higher resistance. KB3D’s 120 V, 750 W silicone heater works out to 120² / 750 = 19.2 Ω, more than 7 times the Ender 3 bed’s 2.62 Ω. Measure a mains bed only with the printer unplugged from the wall. Leave mains wiring to a licensed electrician.

How to measure heater resistance safely

1. Turn the printer off and unplug it. Prusa’s procedure starts with the printer powered down and at room temperature.
2. Disconnect the heater from the board. Prusa says the part “has to be unplugged from the mainboard.” Fluke notes that anything wired in parallel usually lowers the reading.
3. Set the meter to resistance. Use the lowest range that covers 20 Ω, as Prusa advises.
4. Zero the probes. Touch them together and note the reading, or turn on the meter’s REL (zero) mode. Enter the reading in “Reading with the probes touching.”
5. Probe the heater’s two wires. Prusa measures at the screws in the connector for both heaters, or at the bed’s solder pads or the screws holding its cables.
6. Compare with the calculator. Prusa calls the reading “only indicative,” since resistance can shift by tens of percent with temperature. A cold reading inside the range confirms the rating. One far outside it means the wrong part or a fault.