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Battery Life Calculator — Runtime, Capacity + Charge Time

Capacity ÷ load × efficiency = runtime — mAh and Wh modes, reverse-solve capacity, charge time, share link — browser-only

  • Runs locally
  • Category Calculator
  • Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Estimate how long a battery lasts from its capacity and the current it feeds. Switch to Wh/W for power banks and laptops, reverse-solve the capacity you need, or estimate charge time.

Quick examples

Usable fraction of nameplate capacity. 0.8 keeps headroom for losses and aging; 1.0 is the lab ceiling.

Estimated runtime
16 h
16 h

Real devices run shorter than this. Efficiency below 1.0 already trims for converter losses and aging, but spiky loads, cold temperature, and an old pack can shave off more.

What this tool does

Free battery life calculator that turns a capacity and a current draw into a runtime you can trust. The core formula is capacity(mAh) divided by load current(mA), multiplied by an efficiency factor that defaults to 0.8 so the estimate keeps headroom for converter losses, cell aging and cold weather. A 2000 mAh pack feeding a steady 100 mA lasts about 16 hours at that 0.8 factor, or 20 hours at the lab ceiling of 1.0. Switch to Wh and watts to size power banks and laptops, reverse the math to find the capacity you need for a target runtime, or estimate constant-current charge time from capacity over charge current. Every result copies in one click and the shareable URL reopens the exact estimate. Runs entirely in your browser with nothing uploaded.

Tool details

Input
Numbers
The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output
Live result + Copy
The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy
Browser-side processing
The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share
Shareable URL state
Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget
Initial JS <= 9 KB
No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit
Calculator · Developer
Category and role tags drive related tools, internal links, and quick fit checks.

How to use

  1. 1. Input

    Paste or drop your content into the tool panel.

  2. 2. Process

    Click the button. All processing is local in your browser.

  3. 3. Copy / Download

    Copy the result or download to disk in one click.

How Battery Life Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

  • Getting a realistic range before a purchase, plan, workout, or schedule decision.
  • Comparing scenarios by changing one input at a time.
  • Turning rough assumptions into a number you can discuss.

Calculation checks

  • Double-check units, dates, rates, and rounding assumptions.
  • Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
  • Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

  1. 1 Ohm's Law Calculator Enter any two of voltage, current, resistance, or power — get the other two plus the formula used — with mV/V/kV, µA/mA/A, Ω/kΩ/MΩ, mW/W/kW prefixes — browser-only. Open
  2. 2 Voltage Drop Calculator Wire and cable voltage drop from current, run length, conductor size and material, with the 3% limit check, copper vs aluminium, single-phase and three-phase, browser-only Open
  3. 3 Unit Converter Convert between length, weight, temperature, area, volume, speed, time — instant, browser-only Open

Real-world use cases

  • Size a battery for an IoT sensor that must run for months

    You are building a soil-moisture sensor that wakes every minute, reads, transmits, and sleeps. The averaged draw works out to 100 microamps in sleep plus brief 30 mA radio bursts, call it 0.5 mA average. Switch to capacity-needed mode, set the target runtime to the months you want in hours, and the tool tells you the mAh you must source. Now you know whether a coin cell carries the design or you need an 18650.

  • Compare two power banks before a long flight

    One power bank prints 20000 mAh, another prints 74 Wh. They are not the same unit. Convert your need to Wh mode, enter the laptop's 12 W draw, and read the hours each can deliver. The Wh number is the honest one for comparison because mAh hides the cell voltage. Now the bigger sticker does not automatically win.

  • Decide if a drone battery survives the planned mission

    Your quad pulls about 18 A in hover from a 5200 mAh pack. Enter 5200 mAh and 18000 mA, keep efficiency near 0.8 to allow for sag and the reserve you never fully use, and the tool gives roughly 14 minutes. If the route needs 16, you know to swap to a bigger pack or trim the payload before you are over the field with a dying battery.

  • Estimate charge time for an overnight top-up

    You plug a 4000 mAh phone into a 2000 mA charger and want to know if it finishes before you wake. Charge mode gives the ideal 2 hours, and the reality note reminds you the constant-voltage taper at the end adds 20 to 40 percent, so plan for closer to 2.5 to 3 hours for a true 100 percent.

Common pitfalls

  • Using the peak current instead of the average. A device that idles at 5 mA but spikes to 200 mA for a half-second every minute does not draw 200 mA continuously. Feed the time-weighted average, not the worst-case peak, or the runtime comes out far too short.

  • Mixing mAh with watts or Wh with milliamps. mAh pairs with mA, Wh pairs with W. Crossing them gives a meaningless number because mAh leaves out the cell voltage. Pick the mode that matches the unit on your spec sheet.

  • Trusting the nameplate capacity on an old battery. A pack rated 3000 mAh after two years of cycling may only hold 2400. Enter the real measured or derated capacity, or lower the efficiency factor, so the estimate reflects the cell you actually have rather than the one on the box.

Privacy

Every number here is computed by plain JavaScript inside your browser tab. The capacity, load, efficiency and charge current you type never leave the page, and nothing is logged or uploaded. The one thing to know: the shareable link encodes your inputs in the URL query string, so a link pasted into chat records those values in the recipient server's access log. That is harmless for a battery estimate, but if you would rather not share the numbers, use the copy button and paste the text instead of the URL.

FAQ

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Made by Toolora · 100% client-side · Updated 2026-05-30