Car Speed Calculator

🏎️ Speed = Distance ÷ Time

Formula: Speed = Distance ÷ Time. Enter any two values and select the unit for each - the calculator works out the speed (or, using the other tabs, distance/time) automatically.

Trip Details

Distance Travelled

Distance Unit

Time Taken

Time Unit

Average Speed - Multi-Leg Journey

Important: Average speed for a trip is total distance ÷ total time - NOT the average of the individual leg speeds. A trip done partly at 60 km/h and partly at 120 km/h does NOT average to 90 km/h unless the TIME spent at each speed is equal. Add each leg of your journey below.

🛣️ Trip Segments

🛑 Stopping Distance Calculator

Formula: Stopping Distance = Reaction Distance + Braking Distance. Reaction distance = speed × reaction time. Braking distance = speed² ÷ (2 × deceleration) - deceleration depends on road conditions.

Speed & Reaction Time

Vehicle Speed

Speed Unit

Reaction Time (seconds)

Driver State

🛣️ Road Conditions

Speed Unit Conversion Table

km/h	mph	m/s	Notes
30 km/h	18.6 mph	8.3 m/s	Residential / school zone
50 km/h	31.1 mph	13.9 m/s	Urban speed limit (typical)
60 km/h	37.3 mph	16.7 m/s	Common urban limit (US)
80 km/h	49.7 mph	22.2 m/s	Rural / undivided highway
100 km/h	62.1 mph	27.8 m/s	Highway speed limit (common)
110 km/h	68.4 mph	30.6 m/s	Motorway (many countries)
120 km/h	74.6 mph	33.3 m/s	Motorway / Autobahn (recommended)
130 km/h	80.8 mph	36.1 m/s	Some European motorways

Conversion factors: 1 km/h = 0.6214 mph = 0.2778 m/s. 1 mph = 1.6093 km/h = 0.4470 m/s. 1 m/s = 3.6 km/h = 2.2369 mph.

🛑 Deceleration Rates by Road Condition

Condition	Deceleration	Friction Coefficient	Notes
Dry asphalt	~7.5 m/s²	≈ 0.75	Good tyres, normal braking
Wet asphalt	~5.0 m/s²	≈ 0.50	Stopping distance increases ~50%
Gravel / loose surface	~3.5 m/s²	≈ 0.35	Reduced tyre grip
Snow	~2.5 m/s²	≈ 0.25	Stopping distance roughly 3× dry
Ice	~1.5 m/s²	≈ 0.15	Stopping distance roughly 5× dry

️ These are general estimates for passenger cars with reasonable tyre condition. Actual stopping distances vary with tyre wear, vehicle weight, brake condition, and road gradient. Always maintain a safe following distance.

Frequently Asked Questions

Speed = Distance ÷ Time. For example, travelling 120 km in 2 hours gives 60 km/h. The key is making sure units are consistent: if time is given in minutes, convert to hours by dividing by 60 (or convert your final answer appropriately). This calculator handles km, miles, and metres for distance, and hours, minutes, and seconds for time - automatically converting and showing the result in km/h, mph, and m/s.

Average speed for a trip = total distance ÷ total time. Simply averaging the speeds of different segments gives equal weight to each speed regardless of how long you travelled at that speed. Example: driving 40 km/h for 2 hours (80 km) then 80 km/h for 1 hour (80 km) - simple average of speeds = 60 km/h, but true average = 160 km total ÷ 3 hours = 53.3 km/h. The true average is always closer to the speed you spent MORE TIME at, not the speed you covered MORE DISTANCE at.

Stopping distance = Reaction distance + Braking distance. Reaction distance = speed (m/s) × reaction time (typically 1.5 seconds for an alert driver). Braking distance = speed² (m/s) ÷ (2 × deceleration), where deceleration depends on road surface: dry asphalt ≈ 7.5 m/s², wet road ≈ 5.0 m/s², ice ≈ 1.5 m/s². Convert km/h to m/s by multiplying by 0.2778 (or dividing by 3.6) before using these formulas.

Braking distance is proportional to the SQUARE of speed (from the physics of kinetic energy: KE = ½mv²). This means doubling your speed roughly quadruples your braking distance - not doubles it. Going from 50 km/h to 100 km/h doesn't just double the distance needed to stop; it increases it by approximately 4×. This non-linear relationship is why even modest speeding can significantly increase collision risk, especially in situations requiring sudden stops.

Reaction distance = speed × reaction time, so it scales linearly with both speed and reaction time. At 100 km/h (27.8 m/s), each additional 0.5 seconds of reaction time adds about 14 metres of reaction distance - before braking even begins. A distracted driver (e.g. using a phone) might have a reaction time of 4+ seconds instead of 1.5 seconds - adding over 60 metres of distance travelled before any braking starts, at highway speed. This is why distraction is so dangerous even without considering braking distance at all.

Wet roads typically reduce the deceleration rate from approximately 7.5 m/s² (dry asphalt) to approximately 5.0 m/s² - a reduction of about 33%. Since braking distance is inversely proportional to deceleration, this increases braking distance by approximately 50%. At 100 km/h, braking distance might increase from roughly 51m (dry) to roughly 77m (wet) - an extra 26 metres. Total stopping distance (including reaction distance, which is unaffected by road surface) increases by a smaller percentage overall, but the absolute increase is still significant at higher speeds.

The most commonly needed conversions: 1 km/h = 0.6214 mph (used when comparing speed limits across countries that use different units). 1 mph = 1.6093 km/h. 1 km/h = 0.2778 m/s (needed for physics-based calculations like stopping distance, since the SI unit for speed is m/s). 1 m/s = 3.6 km/h. A simple approximation: to convert km/h to mph, multiply by 0.6 (slightly underestimates); to convert mph to km/h, multiply by 1.6 (slightly underestimates).

The two-second rule suggests maintaining at least two seconds of following distance behind the vehicle in front - pick a fixed point, and count two seconds between the vehicle ahead passing it and you reaching the same point. This roughly approximates a reasonable reaction distance buffer at typical speeds. However, it does NOT fully account for braking distance, which can be substantial at higher speeds, nor does it adjust for wet, icy, or poor visibility conditions. Many safety organisations recommend increasing to a "four-second rule" in poor conditions or when following large vehicles, to account for the additional braking distance required.

Car Speed Calculator - Speed, Average Speed, and Stopping Distance Explained

Speed calculations seem simple at first glance - distance divided by time - but the details matter more than most drivers realise. A single average speed figure for a journey can conceal large variations in actual driving speed, and the relationship between speed and stopping distance is not linear, which has serious safety implications. This calculator covers three related but distinct calculations: basic speed from distance and time, true average speed across a journey with multiple segments at different speeds, and stopping distance based on speed, driver reaction time, and road surface conditions.

The basic formula: Speed = Distance ÷ Time. Travelling 120 km in 2 hours gives 60 km/h. The key practical point is unit consistency - if your time is in minutes, either convert to hours (divide by 60) before dividing, or convert your final answer. Mixing units (e.g. dividing kilometres by minutes without conversion) is the most common source of error in speed calculations.

Why "Average of Speeds" Is Almost Always Wrong

One of the most persistent misconceptions in everyday speed calculations is that average speed for a trip can be found by averaging the speeds of each segment. This is incorrect unless each segment takes exactly the same amount of time. The correct method is always: total distance ÷ total time.

Consider a classic example: a car travels the first half of a journey's distance at 40 km/h and the second half of the distance at 60 km/h. Simply averaging gives 50 km/h - but this is wrong. Because the car spends more time covering the first half (at the slower speed), the true average speed is lower than 50 km/h - approximately 48 km/h. The error arises because averaging speeds gives equal weight to each speed, but the correct average must weight by the time spent at each speed, not the distance covered. This calculator's "Average Speed (Multi-Leg)" tab handles this correctly by summing total distance and total time across all entered segments.

Stopping Distance - Why Speed Matters More Than You Think

Stopping distance consists of two components that behave very differently as speed increases. Reaction distance - the distance travelled during the time it takes the driver to perceive a hazard and begin braking - increases linearly with speed. Double the speed, and reaction distance doubles. Braking distance - the distance travelled while the brakes are actively slowing the car - increases with the square of speed. Double the speed, and braking distance roughly quadruples.

The Stopping Distance Formulas

Reaction distance = speed (m/s) × reaction time (s)
Braking distance = speed² (m/s) ÷ (2 × deceleration)
Total stopping distance = reaction distance + braking distance
Reaction time: 1.5s is the standard assumption for an alert driver
Deceleration depends on road surface - dry asphalt ≈ 7.5 m/s²
1 km/h = 0.2778 m/s for use in these formulas

️ Why This Matters Practically

At 50 km/h: total stopping distance ≈ 27m (on dry roads)
At 100 km/h: total stopping distance ≈ 85m - over 3× further
Doubling speed roughly quadruples the braking portion alone
Wet roads increase braking distance by approximately 50%
Icy roads can increase braking distance by 4–5×
A distracted driver's reaction time can be 2–3× longer, adding significant extra distance before braking even begins

The Square Law - Why Small Speed Increases Have Outsized Effects

Because braking distance depends on the square of speed, the relationship between speed and stopping distance is non-linear in a way that is easy to underestimate. Going from 50 km/h to 60 km/h - a 20% increase in speed - increases braking distance by approximately 44% (1.2² = 1.44). Going from 100 km/h to 120 km/h - also a 20% increase - adds the same proportional increase to a much larger base distance, resulting in a far larger absolute increase in metres. This is why speed limit increases on highways have disproportionately large effects on the distances required for safe stopping, and why even modest speeding can meaningfully increase the distance needed to avoid a collision.

Practical Applications - Trip Planning and Safe Following Distances

Beyond academic interest, these calculations have direct practical uses. Trip planning benefits from understanding that average speed for a long journey is rarely close to the speed limit - traffic, fuel stops, and varying road types all reduce the effective average below the maximum permitted speed. For safety, understanding stopping distance at your actual driving speed - not just the speed limit - helps establish an appropriate following distance. The commonly cited "two-second rule" (maintaining at least two seconds of following distance) is a simplified heuristic that approximates the reaction distance component but does not fully account for the additional braking distance required, particularly at higher speeds or in poor conditions.

How this calculator works, and where the numbers come from

The Car Speed Calculator applies the standard formula for this calculation to the values you enter and updates the result as you type. The calculation itself happens in your browser, and the page explains the method so you can check any result by hand.

Please note: Results are provided for general information and are calculated from the values you enter.

Sources and further reading

Last reviewed: October 6, 2026 by the CalcQube Editorial Team. See our editorial policy for how we build and check calculators, or report an error.

🏎️ Speed = Distance ÷ Time

Average Speed - Multi-Leg Journey

Segment Breakdown