Runtime calculator
Two multiplications and a division separate a marketing claim from a runtime you can plan around. Put your pack in and see all of it.
By Scooter M. · Published August 31, 2026
- Energy in the pack
- 38.5 Wh
- (5200 mAh ÷ 1000) × 7.4 V
- Most the pack can deliver
- 18.5 W
- 7.4 V × 2.5 A
- Shortest possible runtime, at that ceiling
- 2h 05m
- The floor under every runtime claim for this pack.
- Runtime at the draw you entered
- Not derivable
- No draw entered — and almost no manufacturer publishes one.
- The draw the advertised runtime assumes
- 3.8 W
- 38.5 Wh ÷ 10 h. Compare that with the ceiling above — if it is far below, the advertised figure is the lowest setting.
- Flying with it
- Under 100 Wh — carry-on, no approval needed
- Spare packs always go in carry-on, never checked. Airlines may be stricter than the federal floor, so confirm with your carrier.
Everything above is a ceiling, not a promise. Cold reduces the usable capacity of a lithium pack, every charge cycle takes a little more, and wind and movement make the controller work harder. Real runtime lands below these figures. Nothing above them is physically available.
What each output means
Energy is the honest measure of what a pack holds. Milliamp-hours on their own are only half of it — two packs can both say 5,000 mAh and hold very different amounts of energy if they run at different voltages. Watt-hours is the number that lets you compare anything to anything.
The delivery ceiling is the pack’s own published output current times its voltage. A garment cannot draw more than its battery will give, so this sets the shortest runtime physically available from that pack — the floor under every “up to” claim in the category.
The implied draw is the useful one, and it is the reason this tool exists. Manufacturers do not publish wattage, but they do publish runtimes. If a maker says a 37.6 watt-hour pack lasts “up to 10 hours”, that claim assumes a draw of 3.8 watts. Compare that with the pack’s 18.5-watt ceiling and the shape of the advertising becomes obvious: the headline number describes a trickle.
Why these are ceilings rather than promises
Three things push real runtime below the arithmetic, and all three push the same way. A lithium pack delivers less usable capacity when it is cold, which is unfortunate for a product only used in the cold. Every charge cycle takes a little capacity permanently. And because the controller holds a target temperature rather than a fixed draw, wind and movement make it run the element a larger share of the time.
The manufacturer Dr. Warm, writing about its own products, describes runtime claims as a roof rather than a promise, and puts the lab-to-field and wind-chill gaps in the tens of percent. We agree, which is why every figure here is presented as an upper bound.
Getting the voltage right
The one input people get wrong. In the US, lithium tool packs are commonly labeled with their peak charge voltage rather than their working voltage. DEWALT states it on its own packaging: “Maximum initial battery voltage (measured without a workload) is 20 volts. Nominal voltage is 18.”
So enter 18 for a 20V MAX pack and 10.8 for an M12, not the number on the label. Consumer heated apparel packs quote 7.4 V, which is already the nominal figure, so those you can enter as printed. Wattage explained works through the whole convention.
Frequently asked
- How do I calculate heated jacket runtime?
- Convert the pack to watt-hours — capacity in milliamp-hours divided by 1,000, times nominal voltage — then divide by the watts the garment draws. Manufacturers publish the first two and almost never the third, so the calculator also runs the advertised runtime backwards to show you the draw it assumes.
- How do I convert mAh to watt-hours?
- Divide milliamp-hours by 1,000 to get amp-hours, then multiply by nominal voltage. A 5,200 mAh pack at 7.4 V is 5.2 × 7.4 = 38.5 watt-hours.
- What voltage should I enter for a 20V MAX battery?
- 18 volts. DEWALT prints the distinction on its own packaging: maximum initial voltage measured without a workload is 20 volts, nominal is 18. Using the label figure overstates the energy by about 11%. M12 is 10.8 V nominal for the same reason.
- Why does the calculator ask for the advertised runtime?
- Because it is the most useful input available. Nobody in this category publishes a wattage, but if a maker advertises hours and publishes a capacity, the hours state the draw. Dividing energy by advertised hours gives you the wattage the claim assumes — usually a very low number.
- Is my heated jacket battery allowed on a plane?
- Almost certainly. The FAA limit is 100 watt-hours for unrestricted carry-on, and every mainstream heated apparel pack converts to well under that — the largest published capacity in the category is about 75 Wh. The calculator tells you which band your pack falls in.
- Why is my real runtime shorter than the calculator says?
- Because the calculator gives you a ceiling. Cold reduces the usable capacity of a lithium pack, every charge cycle takes a little more, and wind and movement make the controller run the element harder. Real runtime lands below the arithmetic; nothing lands above it.
Sources
Related
Battery & Power
The conversion nobody publishes: capacity and voltage into watt-hours, watt-hours into hours. Plus the calculator that does it for you.
Watts, volts and the number on the label that is not quite true
Volts, amps, watt-hours and the labeling convention that overstates every tool-battery jacket by 11%.
Flying with heated clothing, and the unit mismatch that makes it confusing
The FAA writes the rule in watt-hours; makers print milliamp-hours. The conversion table, and where every pack lands.
What it costs to run heated clothing
0.71 cents a charge at the EIA average rate. The full table, the space-heater comparison, and the cost that does add up.
Heated Jackets
The biggest category and the noisiest claims. We convert capacity and voltage into watt-hours, then into hours at each setting.