How this instrument works
Acceleration is the rate at which velocity changes. Take the speed you ended with, subtract the speed you started with, and divide by the time the change took: a = (v₂ − v₁) ⁄ t. The unit, metres per second per second, says exactly what it means — every second, the velocity gains that many metres per second. A figure of 5.556 m/s² adds 5.556 m/s of speed for each second it is sustained.
The formula returns the average over the interval. A car does not pull with perfectly constant force from a standstill, but the average is what stopwatch-and-speedometer data can actually support, and for most practical questions it is the number you want. The sign carries meaning too: when the final speed is lower than the initial one, the result is negative — braking is simply acceleration pointed backwards.
Dividing the result by 9.80665 m/s² — standard gravity — restates it in g, the scale bodies actually feel. Around 0.5 g is a brisk sports-car launch, 1 g is the push the floor gives you right now, and sustained values much past 5 g are fighter-pilot territory. The unit menu on the result field performs this division for you.
- Enter the initial speed — leave it at 0 m/s if the object starts from rest. The unit menu accepts km/h or mph directly.
- Enter the final speed. The instrument converts everything internally, so the two speed fields need not share a unit.
- Set the elapsed time between the two readings in seconds, or minutes for slower changes.
- Read the acceleration; switch its unit to g0 to see the result as a multiple of standard gravity.
Worked example — 0 to 100 km/h in 5 seconds
The classic car-brochure figure: 0 to 100 km/h in 5.0 seconds. First the unit step: 100 km/h is 27.78 m/s. Then the formula: a = (27.78 − 0) ⁄ 5 = 5.556 m/s². Every second of that launch, the car gains about 27.8 km/h of speed — which is precisely why brochures quote the time this way.
Switch the result field to g0 and the same figure reads 0.567 g — a little over half the acceleration of free fall, roughly what a passenger feels pressed into the seat. For comparison, a pull from rest to 10 m/s in 4 seconds — a decent city-bus getaway — is 2.5 m/s², barely a quarter of a g.
Questions
Is this average or instantaneous acceleration?
Average. The formula divides the total velocity change by the total time, so any variation inside the interval is smoothed out. Instantaneous acceleration is the limit of this ratio as the interval shrinks — the derivative dv/dt — which needs continuous data, not two speedometer readings. When the rate of change is constant, the two are identical.
What does a negative result mean?
The object slowed down. Enter a final speed below the initial one — say 30 m/s down to 0 in 6 seconds — and the instrument returns −5 m/s². Physically that is a deceleration of 5 m/s²; mathematically it is acceleration pointing against the motion. The magnitude is what matters for braking distances and seat-belt loads.
How do I convert m/s² to g?
Divide by 9.80665 m/s², the standard gravity value fixed by international convention in 1901. The result field's g0 unit does this automatically: 5.556 m/s² reads as 0.567 g. The g scale is handy because human tolerance and vehicle performance are usually quoted in it — 1 g is what you feel standing still on Earth.
Can I mix units, like km/h in and m/s² out?
Yes. Each speed field carries its own unit menu, and the instrument converts everything to metres and seconds before dividing. Enter 100 km/h and 5 s and the readout is 5.556 m/s² no matter which units the speeds were typed in. Only the displayed numbers change with the menus; the arithmetic underneath is always SI.
What is a typical car's 0–100 km/h figure in m/s²?
A family car doing it in about 10 seconds averages 2.8 m/s², or 0.28 g. A hot hatch at 5 seconds doubles that to 5.6 m/s². Genuine sports cars near 3 seconds reach 9.26 m/s² — about 0.94 g, close to the practical grip limit of road tyres, which is why quicker launches need downforce or more than two driven wheels.
Why does the formula not need a distance?
Because acceleration is defined by velocity and time alone. Distance enters through a different kinematic relation — for a constant rate, d = v₁t + at²⁄2. If what you have is a distance and a time rather than two speeds, that calls for a different instrument; this one wants the speed pair.