How We Calculate

Our approach

A calculator is only useful if you can see how it reached its answer. Each GarageCalcs page explains its method in plain language, shows the formula where there is one and says which numbers are typical examples that you should replace. We use published formulas and standard practice where they exist, we label rules of thumb as rules of thumb, and we do not take payment to favor a product.

Engine

Engine displacement in cubic inches is pi divided by 4, times the bore squared, times the stroke, times the number of cylinders, and liters are cubic inches divided by 61.02. Compression ratio is the swept volume of a cylinder plus the clearance volume, divided by the clearance volume. Piston speed in feet per minute is two times the stroke in inches times the rpm, divided by 12. Horsepower is torque in pound-feet times rpm divided by 5,252. The cam, header and cylinder head tools use common published rules of thumb and estimates, and they say so.

Fuel and forced induction

The carburetor CFM tool uses the standard estimate of displacement times rpm divided by 3,456, times volumetric efficiency. Pressure ratio for a turbo is absolute pressure after the compressor divided by absolute pressure before it. The injector and fuel pump tools turn a power target, fuel type and duty cycle into a flow rate, with a margin for safety. These are sizing estimates, not tuning instructions.

Drivetrain, wheels and tires

The gear and speedometer tools combine tire diameter, axle ratio and transmission ratio to give engine rpm at a road speed. Tire diameter is the wheel diameter plus twice the sidewall height, and the sidewall is the section width times the aspect ratio. Backspacing is half the wheel width plus the offset, in the same units. The drag strip tool estimates elapsed time and speed from weight and horsepower with published approximations.

Shop and body

The paint, primer and filler tools turn the area, coats and coverage figures into quantities and add an allowance for waste. The wire gauge tool matches current and length to a conductor size using standard ampacity and voltage-drop practice. Budget and cost tools use typical figures and are meant for planning.

Converters and lookups

The lookup pages collect commonly published specifications for torque, lug and bolt patterns, spark plug gaps, tap and drill sizes, hitch classes and compressor requirements. They are reference values. Check the specification for your own vehicle and parts before you rely on one.

Finders

Each finder asks a few questions, scores your answers against the options and shows the best match, often with a runner-up. Some answers can force a result when a single factor decides it. Each result explains why it fits, what to check and when it is not the right fit.

Limits

All results are estimates and general guidance. Real situations vary, defaults are examples and standards change. Please read our disclaimer and safety notes, and use the Report an Issue form if a result looks wrong.