Ah to mAh Calculator
Bring a deep-cycle, marine, solar or e-bike rating down to the milliamp-hour scale small electronics are specified in, so the two can be compared without a factor of a thousand getting lost.
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Ah to mAh Calculator
Updates as you type- Entered, in Ah
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- Energy at that voltageWh = Ah × V. The only figure that compares across battery types
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mAh = Ah × 1,000 · Wh = Ah × V
How this is calculated
Multiplying by a thousand needs no explanation, so the useful question is why anyone would want to. Three situations produce it, and they are all cross-boundary problems: comparison shopping across categories that label themselves differently, reading a datasheet written for engineers rather than for the product's category, and specifying chargers or protection circuits, which are almost always rated in milliamps regardless of the battery's size.
What matters far more than the conversion is that a large battery's nameplate is a weaker promise than a small one's, in three specific ways.
The rate it was measured at. Lead-acid capacity is conventionally quoted at the 20-hour rate — the charge available when drained gently over most of a day. Pull the same battery down in four or five hours and you will see perhaps 70% of the printed figure, because internal losses climb with current. A 100 Ah battery is a 100 Ah battery only at the rate the label assumed.
How much of it you may spend. Flooded lead-acid is limited to roughly half its capacity per cycle if it is to last; lithium iron phosphate is routinely specified at 80% or more. The nameplate is what the battery holds, not what you can take out of it repeatedly, and a 100 Ah lithium battery and a 100 Ah lead-acid battery are not the same purchase.
How it was wired. Amp-hours do not add in series. Four 12 V 100 Ah batteries wired in series make a 48 V 100 Ah bank; wired in parallel they make a 12 V 400 Ah bank. Both store the same 4,800 Wh, which is the clearest demonstration of why watt-hours are the honest unit for a bank and amp-hours are only meaningful once someone states the voltage alongside them.
Worked example
One full calculation with real numbers, so you can follow along and check the tool by hand.
A 100 Ah deep-cycle battery, the reference size for off-grid and marine work. Multiplied out, that is 100,000 mAh — a figure you will never see printed, but the one that puts it on the same scale as everything in a drawer of consumer electronics.
Set it against a 3,000 mAh phone battery and the ratio is 33 to one. One deep-cycle battery holds the charge of thirty-three phones. That is a genuinely useful piece of intuition, and it is also the point at which the comparison starts misleading you.
Because the two are not at the same voltage. The deep-cycle battery runs at a nominal 12 V, giving 100 × 12 = 1,200 Wh. The phone runs at 3.85 V, giving 3 × 3.85 = 11.55 Wh. In energy — the quantity that determines what either can actually do — the deep-cycle battery is not thirty-three times the phone. It is closer to a hundred times.
Now apply the discount that a large battery carries and a small one does not. If it is flooded lead-acid, half of that 1,200 Wh is off limits if the battery is to see a reasonable number of cycles, leaving about 600 Wh to spend. That is still fifty-odd phone charges' worth of energy, and it is half of what the nameplate implied.
Three figures for one battery, then: 100,000 mAh on the label, 1,200 Wh of stored energy, and roughly 600 Wh you can actually use. The conversion produces the first. Only the third answers "will this run my equipment overnight", and getting from one to the other needs a voltage, a chemistry and a load — none of which a unit conversion knows anything about.
Visual comparison
| Category | Bar | Typical capacity (Ah) | Energy stored |
|---|---|---|---|
| AA alkaline cell | 2.5 | 4 Wh at 1.5 V | |
| Smartphone | 4 | 15 Wh at 3.85 V | |
| Laptop | 6 | 68 Wh at 11.4 V | |
| USB power bank | 20 | 74 Wh at 3.7 V | |
| Car starter battery | 60 | 720 Wh at 12 V | |
| Deep-cycle or solar battery | 100 | 1,200 Wh at 12 V |
Source: The same category midpoints the reverse page plots, divided by a thousand and paired with their energy.
Reference tables
Both units appear side by side here because that is the comparison this page's readers are making. You have a figure in amp-hours and you want to know how it sits against something specified in milliamp-hours, so the table does the conversion for every category and leaves you to read across rather than convert twice.
Scan the two capacity columns and the labelling problem becomes visible as a shape rather than an argument. Everything above the e-bike row is normally sold in amp-hours; everything below it in milliamp-hours; and the numbers in the two columns overlap heavily across that boundary. A 14 Ah pack and a 20,000 mAh bank sit within touching distance on the mAh column while their printed specifications look nothing alike.
Then read the energy column, which reorders things again. It is the only column in which the categories line up the way intuition expects, because it is the only one that accounts for voltage. If you are choosing between two batteries rather than simply converting one, that column is the comparison and the other two are working.
All ranges are approximate and vary by model, chemistry and age. Nothing here is a substitute for the label on the battery in front of you, and none of it accounts for the share of capacity a given chemistry lets you actually use.
| Category | Typical rangeAh | The same rangemAh | NominalV | Typical energyWh |
|---|---|---|---|---|
| Wireless earbud case | 0.3 – 0.8 | 300 – 800 | 3.7 | 1.9 |
| AA alkaline cell | 1.8 – 3 | 1,800 – 3,000 | 1.5 | 3.8 |
| 18650 lithium cell | 2 – 3.5 | 2,000 – 3,500 | 3.7 | 11.1 |
| Smartphone | 3 – 5.5 | 3,000 – 5,500 | 3.85 | 15.4 |
| Drone or RC LiPo pack | 1.5 – 6 | 1,500 – 6,000 | 11.1 | 33.3 |
| Laptop | 4 – 8 | 4,000 – 8,000 | 11.4 | 68.4 |
| USB power bank | 5 – 27 | 5,000 – 27,000 | 3.7 | 74 |
| E-bike battery | 10 – 20 | 10,000 – 20,000 | 36 | 504 |
| Car starter battery | 45 – 75 | 45,000 – 75,000 | 12 | 720 |
| Deep-cycle or solar battery | 50 – 300 | 50,000 – 300,000 | 12 | 1200 |
Common mistakes
Reading a starter battery’s CCA as its capacity
A car battery advertised at 750 is almost always quoting cold cranking amps, which measures how hard it can push for thirty seconds at −18 °C. Its actual capacity is nearer 60 Ah. The two figures describe opposite abilities and one is not convertible into the other.
Paralleling a new battery with an old one
The pair settles to the behaviour of the weaker cell. A tired battery in parallel with a fresh one drags the fresh one down and takes charge current it cannot hold, so you end up with two mediocre batteries rather than one good one. Replace banks as a set.
Forgetting what the cold does to it
Lead-acid loses roughly a fifth of its capacity near freezing and rather more below that. Lithium is worse in one specific way that matters more: charging it below 0 °C plates the anode and causes permanent damage, which is why cold-climate installations need a battery with a heater or a charger that refuses.
Sizing DC cable from the battery rather than the load
The conductor carries whatever the load draws, not the bank’s capacity. What makes this bite at 12 V is that the currents are large and the voltage budget is tiny — an inverter pulling 100 A over a short run can lose several percent, which is a lot when you only have twelve volts to lose it from.
Charging it with whatever was in the van
Charge current is a fraction of capacity, and the right fraction depends on the chemistry: lead-acid wants roughly a tenth to a fifth of its amp-hour figure, lithium tolerates far more. Too small a charger never finishes the absorption stage, which quietly sulphates a lead-acid bank over months.
This tool provides planning estimates. Always verify final values against your local code and a licensed electrician.
Frequently asked questions
How many mAh is a 100 Ah battery?
100,000 mAh, exactly. The multiplication is definitional — milli- means one thousandth — so no conditions attach to it. What does not follow is that a 100 Ah battery is simply thirty-three phone batteries. It stores thirty-three times the charge, but at 12 V rather than 3.85 V it stores about a hundred times the energy, and energy is the quantity that determines what a battery will actually do.
Why are larger batteries specified in Ah rather than mAh?
Because the milliamp-hour figure becomes unwieldy. A deep-cycle battery in milliamp-hours is a six-digit number that nobody can compare at a glance, while in amp-hours it is a tidy two or three digits. The convention broke along category lines for readability alone — there is no technical distinction between the units, and no point at which one stops being appropriate and the other starts.
Can I compare an e-bike battery to a power bank this way?
Converting both to the same unit is the necessary first step, but it is not sufficient. A 14 Ah e-bike pack is 14,000 mAh and a large power bank is 20,000 mAh, which makes the power bank look bigger. It is not: the e-bike pack runs at 36 V and holds about 504 Wh, while the power bank at 3.7 V holds about 74 Wh — nearly seven times less. Whenever two batteries differ in voltage, convert to watt-hours before drawing any conclusion.
How much of a 100 Ah battery can I actually use?
It depends on the chemistry, and the difference is large enough to change what you buy. Flooded lead-acid is conventionally limited to 50% depth of discharge for reasonable cycle life, so a 100 Ah battery yields about 50 Ah. AGM tolerates a little more. Lithium iron phosphate is routinely specified at 80% and often more, so a 100 Ah lithium battery genuinely delivers 80 Ah or better — which is why a lithium bank of the same usable capacity is physically smaller and lighter than a lead-acid one, and why comparing the two on nameplate amp-hours alone favours the wrong one.
Does wiring batteries in parallel double the mAh?
Yes. Two 100 Ah batteries in parallel make a 200 Ah bank at the original voltage, so 200,000 mAh. Wire the same two in series instead and you get 100 Ah at double the voltage. Both arrangements store identical energy and suit different systems — series to reach an inverter’s required input voltage, parallel to extend capacity at a voltage you already have.
How do I turn amp-hours into runtime?
Multiply by the nominal voltage to get watt-hours, then divide by the load in watts. A 100 Ah 12 V battery holds 1,200 Wh, so a 100 W load runs about 12 hours in theory — and rather less in practice, once depth of discharge, inverter efficiency and the rate penalty are accounted for. Running that calculation properly, in the direction that starts from what you actually consume, is what the battery bank calculator is for.