Solve Speed Distance Time
Last reviewed: January 2026
Solve for speed, distance, or time using the fundamental formula. Supports any consistent unit combination. This calculator runs entirely in your browser — your data stays private, and no account is required.
Speed, distance, and time are related by three simple formulas: speed = distance ÷ time, distance = speed × time, time = distance ÷ speed. Knowing any two values lets you calculate the third.[1] For mixed-speed journeys, calculate each segment separately and sum the times. Driving 100 miles at 65 mph then 50 miles at 35 mph takes 1.54 + 1.43 = 2.97 hours — not the same as dividing total distance (150) by average speed (50), which gives an incorrect 3.0 hours.[2] Average speed for a round trip at different speeds is not the arithmetic mean but the harmonic mean: if you drive 60 mph one way and 40 mph back, the average speed is not 50 mph but 2×60×40/(60+40) = 48 mph, because you spend more time at the slower speed.[3] Use the Pace Calculator for running and cycling speed conversions.
Average speed = total distance ÷ total time. If you drive 120 miles in 2 hours with stops, your average speed is 60 mph even if you never actually traveled at exactly 60 mph. Instantaneous speed is what your speedometer shows at any given moment. For trips with stops, total driving time (excluding stops) gives a different average than total trip time — both are valid for different purposes. When calculating ETA, use total trip time including expected stops.
| Speed | Distance in 1 Hour | Time for 100 Miles |
|---|---|---|
| 30 mph | 30 miles | 3 hr 20 min |
| 55 mph | 55 miles | 1 hr 49 min |
| 65 mph | 65 miles | 1 hr 32 min |
| 75 mph | 75 miles | 1 hr 20 min |
The three variables — speed, distance, and time — are connected by one of the most fundamental equations in physics: distance = speed × time, often written as d = s × t. This equation can be rearranged to solve for any variable: s = d/t for speed, and t = d/s for time. Though simple in concept, this relationship governs everything from daily commuting calculations to spacecraft trajectory planning. Understanding the unit relationships is critical: if distance is in miles and time is in hours, speed must be in miles per hour. Mixing units (kilometers for distance but miles per hour for speed) produces incorrect results unless a conversion factor is applied.
Average speed differs from instantaneous speed in an important way. A car that travels 120 miles in 2 hours has an average speed of 60 mph, but its instantaneous speed varied continuously — accelerating from stops, slowing for curves, and cruising on highways. Average speed always equals total distance divided by total time, regardless of speed variations during the trip. This distinction matters in physics, law enforcement (radar measures instantaneous speed, not average), and logistics where arrival time estimates depend on average speed across varying road conditions.
Trip planning relies on speed-distance-time calculations adjusted for real-world conditions. Highway driving averages 55-65 mph including slowdowns, city driving averages 20-30 mph with traffic lights and congestion, and rural roads fall somewhere between depending on terrain and speed limits. A 300-mile trip that appears to take 4.5 hours at 65 mph realistically takes 5-5.5 hours when accounting for fuel stops, construction zones, and the first and last 30 minutes through urban areas at lower speeds. Professional truckers and logistics companies use sophisticated versions of these calculations, factoring in mandatory rest periods (11 hours of driving maximum per 14-hour window under US DOT rules), seasonal weather patterns, and delivery-window constraints.
Aviation uses the speed-distance-time formula with additional variables. Airspeed (the plane's speed through the air) differs from ground speed (the plane's speed over the earth's surface) because of wind. A headwind of 100 mph reduces an aircraft's ground speed by 100 mph, which is why eastbound transatlantic flights are typically 45-60 minutes shorter than westbound flights — the jet stream (a high-altitude wind belt) provides a significant tailwind in the east direction. Flight planning computers continuously recalculate arrival times based on actual winds encountered, sometimes routing flights hundreds of miles off the direct path to take advantage of favorable winds or avoid strong headwinds.
The most common speed units and their relationships: 1 mile per hour (mph) = 1.609 kilometers per hour (km/h) = 0.447 meters per second (m/s) = 0.869 knots. Knots (nautical miles per hour) are used in aviation and maritime navigation because one nautical mile equals one minute of latitude, making navigation calculations simpler when using coordinate-based positioning. The meter per second is the SI standard used in physics and engineering. For quick mental conversions: to go from km/h to mph, multiply by 0.62 (or divide by 1.6); to go from mph to m/s, divide by 2.24.
Light speed (299,792,458 m/s or approximately 186,282 miles per second) represents the universal speed limit and serves as a useful reference for astronomical distances. Light from the sun takes about 8 minutes and 20 seconds to reach Earth, meaning we always see the sun as it was 8 minutes ago. The nearest star beyond the sun (Proxima Centauri) is 4.24 light-years away — meaning even at light speed, a message would take over 4 years to arrive. These immense distances are why astronomers use light-years rather than miles or kilometers: the Milky Way galaxy is approximately 100,000 light-years across, a number far more manageable than 5.88 × 10¹⁷ miles.
When two objects move toward each other, their closing speed is the sum of their individual speeds. Two cars approaching each other at 60 mph close the distance between them at 120 mph — a critical concept for head-on collision analysis. When objects move in the same direction, the relative speed is the difference: a car traveling at 70 mph overtaking one at 55 mph closes at just 15 mph, which is why passing on highways feels slow despite both vehicles moving at high absolute speeds. These relative-speed calculations are fundamental to air traffic control (ensuring safe separation between aircraft), naval navigation (intercepting or avoiding vessels), and satellite orbital mechanics (rendezvous and docking procedures).
The classic "trains leaving stations" problem illustrates a practical application. If Train A leaves City X heading east at 80 mph and Train B leaves City Y (300 miles east) heading west at 70 mph at the same time, they will meet when their combined distance traveled equals 300 miles. Combined speed is 150 mph, so they meet after 300/150 = 2 hours, with Train A having covered 160 miles and Train B having covered 140 miles. Variations include staggered departure times, stops along the route, and acceleration/deceleration phases. While the basic formula is simple, these word problems develop the analytical skill of translating real-world situations into mathematical equations — a skill that transfers directly to project planning, logistics optimization, and engineering design.
See also: Distance Calculator · Speed Calculator · Running Pace Calculator · Speed Converter · Projectile Motion Calculator
→ Convert units before calculating manually. Mixing miles with km/h or hours with minutes causes errors. This calculator handles conversions automatically.
→ Average speed ≠ the average of two speeds. Driving 60 mph for 30 miles then 30 mph for 30 miles gives an average of 40 mph, not 45. Use our Average Speed Calculator for multi-leg trips.
→ Account for stops and delays. Real travel time includes fuel stops, traffic, and breaks. Add 10–15% for highway trips, 25–30% for urban driving.
→ Use for fitness pacing. Runners and cyclists can calculate pace from distance and time. A 5K in 25 minutes is an 8:03/mile pace. See our Pace Calculator.
See also: Average Speed · Pace Calculator · Road Trip Cost · Distance Calculator