How do you calculate soak time for heat treatment?

Divide the controlling (thickest) section by your soak rate per inch to get base soak hours, then multiply by your allowance factor. With a 3 in section at 1.5 in/hr that is a 2.0 hr base, and a 10% allowance gives 2.2 hr required soak.

What is the rule of thumb for soak time?

The classic shop rule is roughly one hour of soak per inch of thickness once the part is at temperature. That implies a soak rate of 1 in/hr; running 1.5 in/hr as in this example is a faster, more aggressive schedule suited to good furnace circulation.

Does soak time start when the part or the furnace reaches temperature?

Soak time should start when the slowest, coldest part of the load reaches setpoint, not when the furnace controller does. Counting from furnace setpoint on a heavy load is the most common cause of under-soaking.

Why add a soak allowance percentage?

The allowance covers real-world losses the bare formula ignores: dense load packing, lower furnace recovery, thermocouple lag and process margin. A 10% allowance on a 2.0 hr base adds 0.2 hr to reach 2.2 hr.

What happens if you soak too long?

Over-soaking promotes grain growth, surface decarburization and scale, and wastes furnace hours and gas. More time is not free margin, so size the soak to the section rather than padding indefinitely.

How does section thickness affect soak time?

Soak time scales directly with the controlling section. Doubling thickness from 3 in to 6 in at the same 1.5 in/hr rate doubles the base soak from 2.0 hr to 4.0 hr before the allowance is applied.

Heat Treatment, Furnaces & Thermal Processing calculator

Soak Time Calculator

Soak time is the hold-at-temperature portion of a heat-treat cycle, after the load has reached setpoint, that lets the core of the part equalize to surface temperature and the metallurgical transformation complete. Heat treaters, furnace operators and process engineers use it to size cycle length for annealing, hardening, stress relief and tempering. Get it wrong and you either under-soak (soft cores, incomplete transformation, scrap) or over-soak (grain growth, decarburization, wasted furnace hours). A defensible soak time is the difference between a part that meets hardness traverse and one that fails QC.

What this calculator does

Estimate required soak planning time from section thickness or load basis, soak rate, and allowance.
Use it when heat treat recipes need a quick planning check for soak time at temperature before release.
It computes required furnace soak time by dividing the controlling section thickness by your soak rate per inch, then padding the result with a planning safety allowance.

Formula used

Base soak hours = soak basis thickness or load units ÷ soak rate
Required soak time = base soak hours × allowance factor

Inputs explained

Section thickness at thickest point:
Soak rate per inch of section:
Soak planning safety allowance:

How to use the result

Use it when building a heat-treat recipe or routing card for a new part, or when validating cycle time against the 'one hour per inch' rule of thumb before committing furnace capacity.
The simple thickness-divided-by-rate model assumes a single controlling section and uniform heating; thick non-uniform loads, dense baskets, or low furnace recovery can require longer soaks than the formula predicts.

Current U.S. benchmarks

Industrial electricity averages 8.66 cents per kWh across the U.S. (EIA, Apr 2026), up 5.5% from a year earlier. Energy-intensive steps carry this directly into unit cost.

Common questions

How do you calculate soak time for heat treatment? Divide the controlling (thickest) section by your soak rate per inch to get base soak hours, then multiply by your allowance factor. With a 3 in section at 1.5 in/hr that is a 2.0 hr base, and a 10% allowance gives 2.2 hr required soak.
What is the rule of thumb for soak time? The classic shop rule is roughly one hour of soak per inch of thickness once the part is at temperature. That implies a soak rate of 1 in/hr; running 1.5 in/hr as in this example is a faster, more aggressive schedule suited to good furnace circulation.
Does soak time start when the part or the furnace reaches temperature? Soak time should start when the slowest, coldest part of the load reaches setpoint, not when the furnace controller does. Counting from furnace setpoint on a heavy load is the most common cause of under-soaking.
Why add a soak allowance percentage? The allowance covers real-world losses the bare formula ignores: dense load packing, lower furnace recovery, thermocouple lag and process margin. A 10% allowance on a 2.0 hr base adds 0.2 hr to reach 2.2 hr.
What happens if you soak too long? Over-soaking promotes grain growth, surface decarburization and scale, and wastes furnace hours and gas. More time is not free margin, so size the soak to the section rather than padding indefinitely.

Last reviewed 2026-05-12.