Free Tool
Jump Starter Amp Calculator
"Peak amps" on the box is a marketing number. This sizes a pack by what your engine actually needs to turn over.
Estimates for sizing a portable pack, not a replacement for your manufacturer's battery spec. Cranking-amp requirements scale with displacement, compression, and oil viscosity; always check the owner's manual for the battery's rated CCA.
Why peak amps is the wrong number
Most jump starters advertise peak amps — an instantaneous surge measured for a fraction of a second, under no meaningful load. It is close to meaningless for starting a car. What matters is sustained cranking amps delivered for several seconds while the starter motor turns a cold engine.
A rough working rule: a gasoline engine needs about 150 to 200 cranking amps per liter of displacement in mild weather. Diesels need roughly 1.7× that, because of much higher compression. Cold multiplies the requirement again, since oil thickens and the battery's own output falls.
Packs typically publish peak amps only. As a conversion, sustained cranking output is often around one third of the advertised peak — which is why a "2000 peak amp" pack can still fail to start a cold diesel.
What else decides whether a pack works
- Cable gauge. Thin clamps and cables throttle current regardless of the battery inside.
- State of charge. A lithium pack left in a trunk for a year may be at half capacity. Recharge twice a year.
- Temperature at the moment of use. A pack stored in a cold car delivers less than its rating.
Cranking amps by engine and temperature
The figures below are what the calculator produces, written out so you do not have to run it. Each cell is the sustained cranking amps that engine needs at that temperature; the pack you buy should be rated at roughly three times the figure in peak amps, since that is the number manufacturers print.
| Engine | Above 40°F / 4°C | 20 to 40°F / -7 to 4°C | 0 to 20°F / -18 to -7°C | Below 0°F / -18°C |
|---|---|---|---|---|
| 1.5L 4-cyl (compact) | 275 A≈850 peak | 325 A≈1,000 peak | 400 A≈1,200 peak | 475 A≈1,450 peak |
| 2.0L 4-cyl | 350 A≈1,050 peak | 425 A≈1,300 peak | 525 A≈1,600 peak | 625 A≈1,900 peak |
| 2.5L 4-cyl (sedan) | 450 A≈1,350 peak | 525 A≈1,600 peak | 650 A≈1,950 peak | 800 A≈2,400 peak |
| 3.5L V6 (SUV) | 625 A≈1,900 peak | 725 A≈2,200 peak | 925 A≈2,800 peak | 1100 A≈3,300 peak |
| 5.0L V8 | 875 A≈2,650 peak | 1050 A≈3,150 peak | 1325 A≈4,000 peak | 1575 A≈4,750 peak |
| 6.7L diesel | 2000 A≈6,000 peak | 2400 A≈7,200 peak | 3000 A≈9,000 peak | 3600 A≈10,800 peak |
Model: 175 cranking amps per litre for gasoline, ×1.7 for diesel, then ×1.0/1.2/1.5/1.8 for the four temperature bands. Rounded to the nearest 25 A, because the input precision does not justify a sharper number.
A worked example
A 3.5L V6 SUV that has to start at 10°F: 3.5L × 175 CA/L × 1.0 (gasoline) × 1.5 (cold) = 925 cranking amps. Multiply by three and you are shopping for a pack advertised at about 2,800 peak amps — which is why a pack sold as "1000 peak" is the wrong tool for that job, despite the impressive-sounding number.
The same SUV in a mild climate needs only 625 A, or about 1,900 peak. Climate changes the answer more than most buyers expect.
FAQ
Is peak amps the same as cranking amps? No. Peak is an instantaneous surge measured under no meaningful load, typically around three times the sustained cranking output. Only the sustained figure starts an engine.
Why do diesels need so much more? Compression ratios are roughly twice a gasoline engine's, so the starter fights far more resistance. This model uses ×1.7.
Does a bigger pack hurt anything? No. Oversizing costs money and weight but does no harm, and it buys margin as the pack ages and loses capacity.
Shopping for one? See our jump starter guides and data studies for measured prices and picks.