Flashlight Runtime Calculator

Work out your runtime

How this flashlight runtime calculator works

Three steps, and you can do all of them on paper. The tool above is doing nothing you could not do with a pencil.

watts = lumens divided by 60
milliamps = watts divided by pack volts, times 1,000
hours = capacity at that draw, divided by milliamps

Cells in series add their voltage and keep one cell’s capacity. Cells in parallel keep one cell’s voltage and add capacity. Either way the pack holds the same energy, so the runtime comes out the same — series just pulls fewer milliamps to do it.

Why 60 lumens per watt. The best emitters on a datasheet reach 178 lumens per watt, but that is a bare emitter at 25 degrees on a pulse. A finished flashlight loses output to its driver, its reflector and its own heat. Sixty is the figure that reproduces Streamlight’s published runtimes for a 2xAA light, and it is the number this page uses. If you know the real current draw, switch the selector and enter that instead; it is the honest input and it skips this step.

The flashlight runtime calculator checked by hand against a real light

Streamlight publishes 1.33 hours on high and 7.50 hours on low for a two-cell AA light running 150 and 40 lumens.

High. 150 lumens divided by 60 is 2.5 watts. Two alkaline AA cells in series are 3.0 volts, so 2.5 divided by 3.0 is 0.833 amps, or 833 milliamps. The published curve sits at 1,350 milliamp-hours for 500 milliamps and 1,000 for a full amp, so straight-line between them 833 milliamps gives 1,117 milliamp-hours, not the 2,800 on the packet. 1,117 divided by 833 is 1.34 hours. Streamlight says 1.33. That is within one percent.

Low. 40 lumens divided by 60 is 0.667 watts, which at 3.0 volts is 222 milliamps. At that gentler rate the same cell gives 2,031 milliamp-hours, and 2,031 divided by 222 is 9.14 hours against a published 7.50. The lumens path runs generous at low output, so read a low-mode answer as a ceiling rather than a promise.

The lesson in both lines is the one that surprises people: an alkaline AA does not have a fixed capacity. Pull it hard and it gives up a third of what it gives up slowly. That single fact is why most runtime guesses are wrong.

The reference data behind this flashlight runtime calculator

What a cell actually gives at a given draw

Milliamp-hours delivered down to 0.8 volts per cell, read off each maker’s own discharge curves.

Read the alkaline rows across and the lithium row across, then read them against each other. An alkaline AA loses two thirds of its capacity between a 25 milliamp trickle and a 1 amp pull. A lithium AA loses seven percent over the same range. That is the whole argument for lithium primaries in a light you keep for emergencies.

Printed cell specifications

What emitters actually manage

Datasheet figures at the test conditions each maker states. Use them as a ceiling. If a light’s claimed lumens and claimed runtime together need more than 178 lumens per watt, the claim cannot be met by any white emitter you can buy, and the number is a peak reading rather than a sustained one.

What the cold does

Share of room-temperature capacity, from the makers’ temperature curves. This is the table that decides what goes in a winter coat pocket: an alkaline AA pulled at a quarter of an amp keeps nine percent of its capacity at minus four, and a lithium AA keeps nearly all of it.

What this flashlight runtime calculator will not do

It will not guess your light’s real current draw, because almost nobody publishes it. It will not model a step-down schedule, so a light that drops out of turbo after two minutes will run longer than this answer says and be dimmer while it does it. It will not tell you how far the beam throws, which is candela, not lumens. And it stops rather than extrapolating below 25 milliamps or above the last published point on a cell’s curve.

What people ask about the flashlight runtime calculator

Why does my flashlight never last as long as the box says?

Two reasons. The published runtime is usually measured to the point where output falls to a tenth of the starting level, so most of that time is spent dim. And on alkaline cells the capacity collapses under load: a cell that holds 2,840 milliamp-hours at a trickle holds 1,000 at a full amp.

Do lithium AA cells really last longer?

At low draw, only a little. At high draw, enormously. The tables above show a lithium AA holding 3,270 milliamp-hours at 1 amp where an alkaline holds 1,000. In the cold the gap is wider still.

Is a bigger number of lumens better?

Rarely. Most jobs are done at under 100 lumens, and a high setting burns the battery and your night vision together. Buy the light for its low mode and its beam shape.

Does wiring cells in series or parallel change the runtime?

No. The pack holds the same energy either way. Series raises the voltage so the light draws fewer milliamps; parallel keeps the voltage low and draws more. The hours come out the same.

Why does the flashlight runtime calculator give a different answer when I enter current instead of lumens?

Because the lumens path has to assume how efficient your light is, and this page assumes 60 lumens per watt. The current path assumes nothing.

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Sources

Capacity and temperature curves: product datasheets from Energizer (E91, E92, L91), Duracell (MN1500), Panasonic (eneloop, eneloop pro, CR123A, NCR18650B, NCR18650GA), Samsung SDI (INR18650-30Q, INR21700-50E), Molicel (INR18650-P26A, INR21700-P42A) and XTAR. Emitter figures: Nichia, Cree and Luminus datasheets at the test conditions shown. Published flashlight runtimes: Streamlight, Olight, Fenix and Nitecore product pages. Where a maker publishes a curve rather than a table, the figure is read off the curve and is good to a few percent.

Data updated September 2026. Every capacity figure re-checked against the maker’s published datasheet on 15 September 2026.