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Every cordless tool owner has asked the same question halfway through a job: how long will this battery actually last? This battery runtime calculator gives you a straight answer. Enter your platform voltage, your battery’s amp-hour (Ah) rating and the tool you are running, and the battery runtime calculator estimates the working time per pack — plus how many packs a full day on that tool would take.
The figures are estimates, not promises. Tool draw changes with material, blade sharpness and how hard you push, so the calculator reports a range rather than one false-precise number. Your tool’s manual and the gauge on the pack always win.
How the battery runtime calculator works
The maths is the same three steps you would do on paper. First, the calculator turns your pack into energy: volts × amp-hours = watt-hours. An 18V 5.0Ah battery holds 90 watt-hours on paper. Second, it takes 15 percent off that figure — usable Wh = V × Ah × 0.85 — because no pack gives up its last drop, and motors, gearboxes and heat all take a cut. That 0.85 is a reserve and drivetrain allowance, stated openly so you can judge it. Third, it divides the usable energy by what the tool actually draws across the hour: runtime = usable Wh ÷ (tool watts × duty factor).
Duty cycle is the honest part most runtime claims skip. A circular saw cutting continuously draws its full load; a drill driving screws draws power in bursts. The duty selector stands in for that reality — light at 25 percent trigger time, intermittent at 50 percent, heavy at 75 percent — and the result always shows the full light-to-heavy range, so you can see the best and worst case side by side. Packs per day is then simple division: packs = your work hours ÷ runtime per pack, rounded up.
One voltage note: for a 36V two-pack tool, enter the Ah of each 18V pack. Two 5.0Ah packs paired for 36V hold the same energy as 36 × 5.0 — the calculator’s voltage setting already accounts for the pair.
Battery runtime calculator: worked examples
Three combinations, worked by hand, so you can check the calculator against your own arithmetic:
| Combination | Usable energy | Runtime at chosen duty | Range (heavy–light) | Packs for the day |
|---|---|---|---|---|
| 18V 5.0Ah, circular saw 600 W, intermittent (50%), 6-hour day | 18 × 5.0 × 0.85 = 76.5 Wh | 76.5 ÷ 300 W ≈ 15 minutes | About 10–31 minutes | 24 packs’ worth of energy |
| 18V 2.0Ah, drill/driver 250 W, light (25%), 4-hour day | 18 × 2.0 × 0.85 = 30.6 Wh | 30.6 ÷ 62.5 W ≈ 29 minutes | About 10–29 minutes | 9 packs’ worth of energy |
| 54V 6.0Ah, chainsaw 800 W, heavy (75%), 3-hour day | 54 × 6.0 × 0.85 = 275.4 Wh | 275.4 ÷ 600 W ≈ 28 minutes | About 28–83 minutes | 7 packs’ worth of energy |
Read the saw row carefully: a full day of intermittent sawing eats the energy of 24 five-amp-hour packs. Nobody owns 24 packs — the number is the point. It is why tradies rotate four to six packs through a fast charger, and why a big circular saw job is planned around charging, not just cutting.
Planning a full day on cordless tools
Runtime is a platform decision before it is a battery decision. The packs you own only fit one system, so start with our battery platform compatibility guide if you are choosing or expanding a system — it covers what fits what across Makita LXT, Milwaukee M18, DeWalt XR, Ryobi ONE+, Bosch, AEG and Ozito PXC. If you are still deciding which tool does the job in the first place, our drill vs impact driver comparison settles the most common pairing, and the current models are shortlisted in our cordless drill picks and impact driver picks.
Two habits stretch any pack further than a bigger battery does. Keep a second pack on the charger and swap at the first sign of sag rather than wringing a pack dry — deep discharges are what shorten pack life. And match the pack to the tool: compact 2.0Ah packs keep a drill light overhead, while saws and grinders earn their keep on 5.0Ah and larger packs where the runtime maths above stops looking grim.
Keep going: the other Mannacote calculator
Sizing batteries for a generator or portable power station instead? Our generator and power station sizing calculator totals your appliance loads and surge peaks the same way. The two tools share one idea: buy for the load you can add up, not the load you guessed.
And if you are shopping the stations themselves, our best portable power stations Australia guide ranks the current LFP models with prices checked at two Australian retailers.
Frequently Asked Questions
How accurate is this battery runtime calculator?
It is a planning estimate, built to be honest rather than flattering. Real runtime moves with material density, blade or bit condition, temperature and how hard you lean on the tool, which is why the calculator shows a light-to-heavy range instead of one number. Manufacturer runtime claims vary by test setup; your tool’s manual wins any argument.
Why does the calculator take 15 percent off my battery?
Because a pack never delivers its full paper capacity to the cut. Motors and gearboxes waste energy as heat, the tool’s electronics cut out before the cells are truly empty to protect them, and older packs hold less than new ones. The 0.85 factor bundles those losses into one stated allowance.
What does duty cycle mean for a cordless tool?
It is the share of the hour the motor is actually working. A drill driving screws might be under load a quarter of the time; a saw ripping hardwood, three quarters. Choosing a realistic duty cycle matters more to the result than a decimal place of battery capacity.
How many batteries do I need for a full day?
Divide your work hours by the runtime per pack at your duty cycle — the calculator does this for you. In practice, most owners rotate fewer packs than the raw figure suggests because packs recharge while others work; a dual or rapid charger closes most of the gap.
Does a bigger Ah battery make my tool more powerful?
Mostly it buys time, not power: a 5.0Ah pack runs roughly two and a half times longer than a 2.0Ah pack on the same tool. Some high-output pack tiers do feed demanding tools better under heavy load, but the fit is the same — runtime is the main thing capacity changes.