Car battery CCA calculator
How many cold cranking amps your engine needs, and how charged your battery is from its resting voltage.
The manual's figure beats the rule of thumb. The battery's CCA is on its label.
Measure after the engine has been off for at least four hours, and with the lights and accessories off. Nothing you type here is saved to the link.
What the numbers on a car battery mean
Two numbers on a car battery's label matter most: cold cranking amps, for starting, and reserve capacity, for emergencies. This page is about starting the engine. For how long a battery can run a load, use the battery runtime calculator.
What a starting battery does
A car battery must do one hard job, delivering a few hundred amps for a few seconds, and then spend most of its life being topped up by the alternator. That is why it is rated by current for a short time (CCA) and by a modest sustained load (reserve capacity), not by amp-hours as deep-cycle batteries are. This page checks the first against your engine and the second against a voltage reading.
Worked examples
A 5.7 L gasoline engine has 348 cubic inches (1 liter is 61.02 cubic inches), so the rule of thumb gives about 348 CCA; a 6.7 L diesel gives 818. A battery that rests at 12.35 volts is about 63 percent charged by the Battery University table, or 69 percent on the other scale: low enough to recharge it. A battery with a reserve capacity of 120 minutes holds roughly 50 amp-hours at the 25-amp rate.
Common mistakes
Reading the voltage right after driving or charging is the first: the number is too high. Reading a battery below 12.4 V and replacing it is the second: a discharged battery that charges back to 12.6 V may be fine, and the cause of the drain, such as a parasitic load or short trips, may be the real problem. Third, comparing CCA with CA or with marine cranking amps, which are measured at 32 °F and read larger, so the numbers are not comparable. Fourth, matching on CCA alone and ignoring the group size and terminal layout. Finally, ignoring age: a battery loses capacity over its life, so a CCA label is a new-battery figure.
Limits of this method
The rule of thumb is a floor derived from retail articles, not from the engineering standards, and we found no published derivation. Starter current depends on the engine's compression, oil viscosity and the starter itself, none of which the displacement captures. The rest-voltage scales were measured on lead-acid batteries of a particular construction. AGM, calcium and enhanced flooded batteries, and any battery in a vehicle with a start-stop system, may differ, and a voltmeter cannot tell a battery that holds charge from one that cannot deliver current. A load test or a conductance tester at a parts store gives that answer. For a no-start or a repeatedly dead battery, have the charging system tested too.
What does CCA mean on a car battery?
Cold cranking amps. It is the current a fully charged 12-volt battery can deliver for 30 seconds at 0 °F (−18 °C) while the voltage stays at or above 7.2 volts, which is 1.2 volts per cell. The test is defined by Battery Council International (BCI) and SAE standards, which we did not open, so we rely on battery makers' and sellers' descriptions of it. A higher CCA means more starting current in the cold, not more total energy. Some sellers also print 'cranking amps' (CA) measured at 32 °F; it is a different, larger number and not comparable to CCA.
How many CCA does my engine need?
The number to follow is the minimum in your owner's manual, or the rating on the battery that came with the car. As a rough sanity check, many battery sellers and repair shops quote about 1 CCA per cubic inch of engine displacement for gasoline engines and twice that for diesels. A 5.7 L gasoline V8 (348 cubic inches) comes to about 348 CCA, and a 6.7 L diesel to about 818. We could not find a standard that states this rule, and modern engines with efficient starters can start on less, so treat it as a floor, not a spec.
Is it bad to buy a battery with more CCA than I need?
No. The engine draws only what it needs, so extra CCA is spare capacity, not a mistake. It costs more, and the battery must still fit the group size, terminal position and hold-down of your car. In a cold climate, extra CCA is a reasonable choice, because a battery loses power as it cools. Going lower than the manual's figure is the risky direction.
How much power does a battery lose in the cold?
Battery University reports that a battery delivering 100 percent at 80 °F (27 °C) delivers only about 50 percent at 0 °F (−18 °C), and that at −4 °F (−20 °C) most batteries are at about 50 percent of performance. That is for batteries in general, not for starter batteries in particular, and the loss at a given temperature varies with the chemistry and the state of charge. A cold engine also turns harder because the oil is thicker, so the demand rises as the supply falls. This is why a battery that works in October can fail in January.
What voltage means a car battery is charged?
At rest, a healthy flooded starter battery reads about 12.6 to 12.7 volts when full. Battery University lists 12.65 V for 100 percent, 12.45 for 75, 12.24 for 50, 12.06 for 25 and 11.89 for 0, measured at 78 °F after a 24-hour rest. A second scale attributed to BCI in a boating magazine reads 12.6, 12.4, 12.2, 12.0 and 11.7 V. At 12.35 V the first table gives about 63 percent and the second 69 percent. The two scales agree within 1 to 2 tenths of a volt, but the percentage between table points is our interpolation.
Why does my battery read 12.8 V right after driving?
Charging leaves a surface charge that raises the voltage for a while. Battery University says a battery should rest in open circuit for at least four hours before you read the voltage, and its own table was measured after 24 hours. Battery University notes that a brief discharge before measuring counteracts the surface charge. Calcium-based plates read 5 to 8 percent higher at the same state of charge, according to the same source.
What is reserve capacity?
Reserve capacity (RC) is the number of minutes a new, fully charged battery at 80 °F can supply 25 amps before the voltage falls to 1.75 volts per cell, or 10.5 volts on a 12-volt battery (BCI standard as described by Cummins and battery makers). It shows how long the battery could run the ignition and lights if the alternator failed. Multiplying by 25 amps and dividing by 60 gives the amp-hours, about 50 Ah for an RC of 120 minutes (our arithmetic). For running appliances from a battery, see the battery runtime calculator on this site.
Rest voltage and charge
| Charge | Battery University | BCI scale (reported) |
|---|---|---|
| 100% | 12.65 V | 12.6 V |
| 75% | 12.45 V | 12.4 V |
| 50% | 12.24 V | 12.2 V |
| 25% | 12.06 V | 12.0 V |
| 0% | 11.89 V | 11.7 V |
Battery University (Cadex, a battery-analyzer maker, so a vendor source) measured its column on a starter battery with antimony plates at 78 °F after 24 hours of rest. The second column is the scale printed by Practical Sailor and attributed to BCI; we did not open a BCI document.
Safety: car batteries hold lead and acid and can release explosive hydrogen gas while charging. Do not charge a frozen or damaged battery, and keep sparks and flames away. Not a substitute for the owner's manual or a battery test. Sources: Battery University BU-903 (state of charge by voltage) and BU-502 (low-temperature capacity), batteryuniversity.com, read October 28, 2026; CCA and reserve-capacity definitions as described by battery makers and sellers and by a Cummins flyer citing BCI. The cubic-inch rule of thumb comes from retail and repair-shop articles and is not a standard. Related: battery runtime calculator, engine displacement calculator, horsepower calculator. How we check our calculators.