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About Cartridge Heater Diameter

If you measure the actual diameter of a cartridge heater, it won't be the same as its specified diameter (i.e., a 1/2" diameter cartridge heater won't measure 0.500"... ever). The reason for that is simple: the heater must fit a (carefully) bored hole.

The typical design process goes something like this (when heating "blocks" of steel, aluminum or other materials):

  1. Determine the total power required;
  2. Figure out how to distribute the power;
  3. Determine the optimal length and diameter to achieve even heating.

Once the diameter of the cartridge heaters has been determined, the holes for the heaters should be bored to that exact diameter. For example, if you decided that 1/2" cartridge heaters are what you need, then you should use a 1/2" drill to bore the receptacle for the heaters. It is important that the drill or boring tool doesn't wander during the process. We have seen cases where heaters that should have given several years of service life lasted mere hours in holes of non-constant diameter.

As always, if you have an industrial process heating task and you're not quite sure about the best heater or how to size them, you can always give us a call at (866) 685-4443, email us at, or fill out one of our contact forms.

Cartridge heater diameter tolerance and other tolerances

The tables below give the maximum and minimum actual diameter of cartridge heaters available here. Heater diameters shown in bold with orange background are Build-to-Order sizes that are available for shipment in as little as 2 days; links in the table are to the respective Stock List shopping pages for the heater diameter. If you're looking for something specific that isn't in our stock list, you can call us and tell us what you need or you can send your cartridge heater specifications to us at and we will quote you within one business day.

Imperial Metric
Nominal diam. (bore/drill size) Actual diam., max. Actual diam., min. Nominal hole diam. Actual diam., max. Actual diam., min.
1/8" (0.125")   0.124" 0.120"    6mm 5.97mm 5.87mm
5/32" (0.15625") 0.155" 0.152" 6.5mm 6.48mm 6.38mm
3/16" (0.1875")   0.186" 0.183"    8mm 7.98mm 7.85mm
1/4" (0.250")   0.249" 0.244"  10mm 9.96mm 9.85mm
5/16" (0.3125")   0.312" 0.308"  12mm 11.96mm 11.86mm
3/8" (0.375")   0.374" 0.369" 12.5mm   12.47mm 12.34mm
1/2" (0.500")   0.499" 0.494"  13mm 12.98mm 12.85mm
5/8" (0.625")   0.624" 0.619"  15mm 14.99mm 14.86mm
3/4" (0.750")   0.749" 0.741"  16mm 15.95mm 15.82mm
15/16" (0.9375")     0.936" 0.928" 17.5mm   17.47mm 17.27mm
  1" (1.000") 0.999" 0.991"  20mm 19.96mm 19.76mm
1-1/4" (1.250")      1.249" 1.241"
1.90" (1.900")    1.910" 1.890"
2-3/8" (2.375")      2.385" 2.365"

Cartridge heater tolerance of power, length, and camber

Power tolerance, or how close the actual dissipated power at the specified voltage is to the specified power, is typically +5%/-10%. It is possible to have cartridge heaters built with tighter tolerances, but it will drive the price up. For instance, we have one customer who uses cartridge heaters as damping resistors in an RF application. For that guy, we specify the resistance and the resistance tolerance of the cartridge heater rather than its power.

Length tolerance is a measurement of what the actual length is relative to the specified length. For our stock list cartridge heaters, length tolerance is ±0.005" per six inches of length for units up to 12" long and ±0.020" per foot for units over 12" long. For quick-turn Build-To-Order cartridge heaters, length tolerance is ±2%.

Camber tolerance is a measure of straightness of the cartridge heater. Cartridge heaters may vary from a straight line by as much as 0.010" per foot for lengths up to 12", and by as much as 0.018" per foot for lengths greater than 12".