How this space travel time calculator works
Every number on this page comes with the same honest asterisk, so let us put it first: this calculator computes straight-line travel at your chosen speed, and real spacecraft cannot fly straight. They coast along curved transfer orbits shaped by gravity and fuel budgets, which is why a real Mars mission takes 6 to 9 months while the straight-line math cheerfully suggests weeks. We compute the fantasy cleanly, label it a fantasy, and tell you what the real mission costs on every result. If you want the physics of why getting anywhere is so hard, the delta-v calculator owns the rocket equation, and it does not sugarcoat.
Pick a destination and a speed, or type your own of either, and you get the travel time, the time light takes to make the same trip, and a reference table of your destination at several iconic speeds. Or flip to the constant 1 g mode: accelerate at Earth gravity to the midpoint, flip the ship, and decelerate, the way science fiction crews do it. That mode handles relativity honestly, because past about 10 percent of light speed the universe insists.
The formula
Flip and burn: t = 2 × √(d ÷ a)
Relativistic flip and burn: ship time = (2c ÷ a) × acosh(1 + a d ÷ 2c²)
In the cruise line, distance and speed are yours to pick, and nothing else is hiding in it, which is exactly why it is a fantasy. In the flip and burn lines, d is the total distance, a is your acceleration (1 g = 9.80665 m/s², the exact standard value), and c is the speed of light, 299,792.458 km/s exactly. The Newtonian middle line works beautifully until the top speed nears light speed, at which point it starts predicting the impossible; the relativistic line never does, and it splits the answer in two: time on the ship's clocks and time on Earth's, which genuinely disagree. Earth time comes from the companion formula t = 2 × √((d ÷ 2c)² + d ÷ a).
Worked example
Mars at its average distance of 225 million km, aboard something moving at the Parker Solar Probe record of 192 km/s, the fastest any human-built object has ever gone: 225,000,000 ÷ 192 = 1,171,875 seconds = 13.56 days. (One honest clause: Parker hit that speed diving past the Sun on a bound orbit, trading gravity for velocity. It is not a cruise our ships could point at a destination.)
The 1 g flip and burn does better: t = 2 × √(225,000,000,000 m ÷ 9.80665) = about 302,943 seconds = 3.51 days, peaking at 1,485 km/s (0.5% of light speed) at the flip. Light itself makes the trip in 12.5 minutes.
And the real mission? 6 to 9 months, on a curved transfer orbit, because gravity is not optional and fuel is not free. The fastest object in human history would need two weeks, the science fiction burn needs three and a half days, and the actual spacecraft needs the better part of a year. Space is not kind to schedules.
Why real spacecraft cannot fly straight
Two reasons, and they gang up. First, gravity is not optional: the moment you leave Earth you are in orbit around the Sun, and every path you can afford is a curve. Second, fuel is finite: the rocket equation charges exponentially more propellant for every extra meter per second of speed change, so missions buy the cheapest curve, not the shortest line. The classic budget move is a transfer orbit: burn once to stretch your orbit out to the target's distance, coast the long arc, and arrive when the target does. That arithmetic is why Apollo took about 3 days to the Moon, why Mars transfers run 6 to 9 months and only make sense during launch windows that open about every 26 months, and why New Horizons needed 9.5 years to reach Pluto despite being the fastest spacecraft ever launched.
The launch window point hides a teaching most calculators skip: Mars is not a place at a fixed distance. It swings between about 54.6 million km at a rare close approach and 401 million km when the Sun sits between you, a factor of seven. Any "time to Mars" answer that does not say which Mars it means is answering a different question. This page defaults to the 225 million km average and shows you the range on every Mars result.
The 1 g fantasy, taken seriously
Constant acceleration is the propulsion physicists doodle and novelists ship: hold 1 g the whole way and the crew gets Earth-normal gravity for free, floor pointed aft, no exercise bike required. Accelerate to the midpoint, flip, decelerate. The times are startling: 3.5 hours to the Moon, 3.51 days to Mars at its average distance, about 18 days to Pluto, 3.54 years to Proxima Centauri, and 28.6 years to the Andromeda galaxy, the last two measured on the ship's clocks. On Earth's clocks those same two trips take 5.87 years and 2.5 million years, and both figures are true at once. This is time dilation, not bookkeeping: the moving ship's clocks genuinely run slower, and the calculator switches to the full relativistic equations whenever a trip gets fast enough to need them. A crew could reach Andromeda within a career; everyone who waved goodbye, and their civilization, and arguably their species, would be 2.5 million years gone. That is the twin paradox with a cargo manifest.
Why nobody flies this way: the rocket equation eats you. Holding 1 g to Mars means a total speed change near 3,000 km/s, and a hydrogen-oxygen rocket, the best chemistry we have, would need a mass ratio around 10292 to buy it. The observable universe holds roughly 1080 atoms. Fusion drives, beamed sails, or something not yet invented might someday shrink that; until then the flip and burn stays the best trip nobody can book, and the delta-v calculator will show you exactly how the equation does the eating.
The light delay is the real distance
Every result on this page shows the light travel time, and it is the most practical number here, because it is also the communications delay: nothing, including your commands and your telemetry, moves faster. The Moon sits 1.28 light-seconds away, which is why Apollo conversations carried that famous small pause. Mars sits 3 to 22 light-minutes away, which is why nobody drives a Mars rover with a joystick: drivers send a day's plan, then wait for the round trip to learn what happened. Proxima Centauri is 4.25 light-years out, so a single question and its answer costs most of a decade. The speed of light preset exists because "how long does light take to get there" is a real question with a clean answer; just remember the answer describes photons, for whom, thanks to relativity, the trip takes no time at all from their own point of view.
Andromeda by car
Set the destination to Andromeda and the speed to a highway car, and the calculator will tell you, deadpan, that the drive takes about 27 trillion years, roughly 2,000 times the current age of the universe. This is the page working as intended. The presets run from a car to light itself precisely so the scale can land: the Moon is a 160 day drive, Mars at Parker speed is two weeks, and the nearest star is about 75,000 years away at Voyager 1's speed. Our road trip calculator models the fuel and bathroom stops on the terrestrial version of this problem; out here the next services are 54.6 million km away at best, and they are not open. Bring snacks.