Boat Speed Calculator
Calculate boat speed from distance and time, estimate propeller speed, use Crouch's formula for top speed and horsepower, or check displacement hull speed.
Boat speed, prop speed, Crouch estimate, and hull speed in one tool
Use the distance and time mode to calculate actual speed over a route. Enter distance in nautical miles, statute miles, or kilometers, then enter elapsed time to get speed in knots, mph, and km/h.
Calculate boat speed by dividing distance by time: speed = distance / time. A boat that travels 30 nautical miles in 2 hours moves at 15 knots.
The propeller mode estimates speed from engine RPM, gear ratio, prop pitch, and prop slip. Crouch mode estimates top speed or required horsepower from power, loaded displacement, and boat type. The hull speed mode estimates the efficient displacement speed from waterline length.
Boat speed varies by hull design, engine power, water conditions, current, wind, and total load. Results are planning estimates, so also consider trim, prop condition, sea state, bottom growth, and engine setup.
Estimated boat speed
-- mph and -- km/h.
Knots
--
Nautical miles per hour.
Miles per hour
--
Statute miles per hour.
Kilometers per hour
--
Metric speed conversion.
Minutes per nautical mile
--
Useful for route timing.
Method
--
Input summary
--
Formula used
--
Note: Results are estimates. Verify speed with GPS, charted distance, engine instruments, and safe operating judgment on the water.
How to use the boat speed calculator
- Choose a method: Use distance and time for a real run, propeller pitch speed for engine and prop setup, Crouch's formula for power and displacement estimates, or hull speed for displacement boats.
- Enter the measurements: Keep the units consistent with the fields. Distance mode accepts nautical miles, miles, or kilometers.
- Add optional current correction: In distance mode, enter a helping current as positive or an opposing current as negative if you want an estimated still-water boat speed.
- Read all speed units: The result shows knots, mph, km/h, and minutes per nautical mile so you can use it for navigation, trip timing, or quick comparison.
- Compare with real conditions: Treat propeller, Crouch, and hull-speed estimates as a baseline because load, hull cleanliness, prop slip, trim, and water conditions can change the final number.
Boat speed formulas
Boat speed is usually expressed in knots because navigation distance is commonly measured in nautical miles. One knot equals one nautical mile per hour, which is 1.15078 mph or 1.852 km/h.
The distance and time formula is the most direct method. Propeller speed estimates use pitch, propeller RPM, and slip. Crouch's formula estimates top speed from shaft horsepower and loaded displacement. Hull speed estimates are mainly useful for displacement hulls and do not represent the top speed of planing boats.
Speed in knots = nautical miles / hours
Prop speed in mph = pitch × prop RPM × (1 - slip) / 1056
Crouch speed in mph = C × √(horsepower / displacement)
Hull speed in knots = 1.34 × √waterline length in feet
For a real-world passage, speed over ground is what the GPS shows. Speed through water is the boat's speed relative to the surrounding water, which can differ when current is helping or opposing the route.
Nautical unit reference: NOAA National Ocean Service - nautical miles and knots.
Crouch constant reference table
Crouch's formula is useful when you want a quick top-speed estimate from horsepower and loaded displacement. The constant is not universal; it changes with hull type, setup, and how efficiently the boat converts power into speed.
| Crouch constant | Typical boat type | Best use | Caution |
|---|---|---|---|
| 150 | Cruisers, passenger boats, average runabouts | Conservative planning estimate for heavier recreational boats. | Do not use dry hull weight if the boat is loaded for a trip. |
| 190 | Light high-speed cruisers, fast runabouts | Useful for lighter boats with stronger power-to-weight ratios. | Prop setup and hull condition can move the real result. |
| 210 | Racing boats | Performance estimate for efficient high-speed hulls. | Small errors in displacement can create large speed differences. |
| 220 to 230 | Hydroplanes, racing catamarans, sea sleds | Specialized high-speed applications with very efficient hulls. | Not appropriate for ordinary cruisers, pontoons, or heavy workboats. |
Compared with a simple Crouch-only calculator, this page also lets you compare the estimate against GPS distance/time speed, propeller pitch speed, current correction, and hull-speed limits.
Boat speed conversion table
| Knots | Miles per hour | Kilometers per hour | Time for 10 nautical miles |
|---|---|---|---|
| 5 kn | 5.75 mph | 9.26 km/h | 2 hr 0 min |
| 10 kn | 11.51 mph | 18.52 km/h | 1 hr 0 min |
| 20 kn | 23.02 mph | 37.04 km/h | 30 min |
| 30 kn | 34.52 mph | 55.56 km/h | 20 min |
| 40 kn | 46.03 mph | 74.08 km/h | 15 min |
What changes real boat speed?
Two boats with the same engine and prop can run at different speeds because water, weight, hull design, and setup all matter. Use the calculator result as a clean baseline, then compare it with measured speed on the water.
Load and balance
Fuel, passengers, gear, live wells, and weight distribution can change planing speed and cruise efficiency.
Propeller setup
Pitch, diameter, blade count, cup, damage, and engine mounting height affect RPM, slip, and acceleration.
Water and weather
Current, tide, wind, chop, swell, and hull fouling can make GPS speed different from still-water performance.
Planning trips with boat speed
For trip planning, calculate speed using the pace that the boat can hold comfortably, not just maximum speed. A fast boat that cruises efficiently at 24 knots may be safer and more predictable to plan at 20 to 22 knots if weather, traffic, or no-wake zones are expected.
Route time
Distance / speed
A 30 nautical mile run at 15 knots takes about 2 hours before stops, slow zones, or docking time.
Reserve margin
Plan below ideal speed
Use a conservative cruise speed for fuel, daylight, and weather planning.
Current check
Separate water from ground
Compare two-way runs to reduce current and wind effects when measuring performance.
How to run a reliable boat speed test
A single GPS reading can be misleading if wind, tide, or current is helping in one direction. For a cleaner result, run a measured course in opposite directions, record the speed both ways, and average the two runs before comparing the number with the calculator.
Prepare the boat
Use the same fuel level, passenger load, gear weight, trim position, and engine warm-up each time you test. Small changes can move the speed result.
Run both directions
Run with and against the current or wind over the same path. Average the two GPS speeds to reduce environmental bias.
Record the setup
Write down RPM, prop pitch, gear ratio, trim, sea state, air temperature, and load so future tests are comparable.
Two-way average: If one run shows 24.8 knots and the return run shows 21.6 knots, the average test speed is (24.8 + 21.6) / 2 = 23.2 knots. Use that average as the more useful still-condition estimate.
Safe speed reference: U.S. Coast Guard Navigation Center - Rule 6, Safe Speed.
Prop slip and setup diagnostic table
| What you notice | Possible meaning | What to check |
|---|---|---|
| High RPM, low speed | Too much prop slip, ventilation, damaged prop, or too little pitch. | Prop condition, engine height, trim, hub slip, pitch, and weight distribution. |
| Low RPM, low speed | Engine may be overloaded or the prop may have too much pitch. | Wide-open-throttle RPM range, bottom growth, fuel quality, load, and prop selection. |
| Good top speed, poor acceleration | Pitch may be high, load may be heavy, or the hull may struggle to plane. | Hole shot, passengers, gear load, trim tabs, prop blade count, and engine mounting. |
| Speed changes after cleaning | Hull fouling or rough running gear was adding drag. | Bottom paint, algae growth, prop surface, transducer drag, and trailer damage. |
Use this table as a troubleshooting starting point, not as a diagnosis. If RPM is outside the engine manufacturer's recommended range, confirm the setup with a qualified marine technician or propeller specialist.
Prop slip reference: Mercury Racing - Prop Slip Calculator and explanation.
Turning speed into ETA, fuel, and range planning
Speed is only one part of a safe route plan. Once you estimate boat speed, use it to calculate travel time, fuel reserve, daylight margin, and whether the route still works if weather forces you to slow down.
ETA
Time = distance / speed
A 42 nautical mile route at 14 knots takes 3 hours before docking, idle zones, waiting, or detours.
Fuel reserve
Plan a margin
Do not plan around an empty tank. Keep a reserve for weather, current, rerouting, idling, and unexpected delays.
Slow-speed case
Test a lower speed
If a 20-knot plan becomes a 12-knot plan in chop, the same route takes 67% longer. Check the slower scenario before leaving.
Travel time = nautical miles / knots
Usable range = fuel available after reserve x nautical miles per gallon
Arrival buffer = daylight or deadline - estimated travel time
Interesting fact
A boat speed of 1 knot means the boat travels 1 nautical mile per hour, and the international nautical mile is exactly 1.852 kilometers. NOAA notes that the U.S. adopted the international nautical mile in 1954, which is why modern marine speed calculations convert cleanly between knots and km/h. Source: NOAA National Ocean Service - nautical miles and knots.
Frequently Asked Questions
What is a boat speed calculator?
A boat speed calculator is a marine planning tool that converts distance and time into speed, usually in knots, mph, and km/h. This calculator also estimates speed from propeller pitch and engine RPM, uses Crouch's formula for top speed or required horsepower, and estimates displacement hull speed from waterline length.
How do I calculate boat speed from distance and time?
Convert the distance to nautical miles, convert the elapsed time to hours, then divide distance by time. For example, 25 nautical miles in 1.5 hours equals 16.67 knots. The calculator also converts that result into miles per hour and kilometers per hour.
What is the difference between speed over ground and speed through water?
Speed over ground is the speed measured across the earth, usually by GPS. Speed through water is how fast the hull moves relative to the surrounding water. Current can make speed over ground higher or lower than the boat's still-water speed, which is why the calculator includes an optional current correction.
How accurate is propeller pitch speed?
Propeller pitch speed is an estimate because it depends on prop slip. A prop with zero slip would move forward one pitch length per revolution, but real propellers always slip in water. Planing boats often show slip in the high single digits to around 20 percent, while heavy boats, setup issues, or ventilation can produce higher slip.
What does hull speed mean?
Hull speed is a traditional estimate for displacement boats based on waterline length. The common formula is 1.34 times the square root of waterline length in feet. It is useful for sailboats, trawlers, and displacement hulls, but it is not a top-speed limit for planing hulls.
What is Crouch's formula for boat speed?
Crouch's formula estimates top speed from shaft horsepower, loaded displacement, and a boat-type constant: speed = C x sqrt(horsepower / displacement). It is useful for rough power-to-weight comparisons, but it should not replace measured GPS testing because hull design, propeller efficiency, trim, and loading can change the real result. This calculator also runs the formula backward to estimate the horsepower needed for a target speed.
Why do boats use knots instead of mph?
Boats use knots because nautical charts and navigation distances are built around nautical miles. One knot means one nautical mile per hour. The calculator still shows mph and km/h so the result is easy to compare with land speed or metric estimates.
How does current affect boat speed?
Current changes speed over ground, even when the boat's speed through water stays the same. A 15-knot boat running with a 2-knot current may show about 17 knots over ground, while the same boat against that current may show about 13 knots. For more accurate performance testing, run the same measured course in opposite directions and average the results.
What is a good cruising speed for a boat?
A good cruising speed is usually the speed your boat can hold comfortably without excessive fuel burn, engine strain, pounding, or spray. For many planing boats, that may be below top speed and often sits where the hull is cleanly on plane. For displacement boats, cruising speed is commonly below hull speed because pushing harder can use much more fuel for only a small speed gain.
Why is my calculated speed different from GPS speed?
Calculated speed can differ from GPS speed because the inputs describe ideal or averaged conditions. GPS speed reflects the actual route over ground at that moment, including turns, current, wind, waves, trim, loading, shallow-water effects, and measurement lag. If the difference is large, check the distance unit, elapsed time, prop slip estimate, gear ratio, and whether you are comparing knots with mph.
Other useful calculators
Road Trip Gas Calculator
Estimate travel fuel cost from distance, MPG, and fuel price.
Pool Pump Run Time Calculator
Estimate daily pool pump runtime from volume and flow rate.
Salt Pool Calculator
Calculate pool salt dosage from volume and salinity.
Kinetic Energy Calculator
Calculate kinetic energy from mass and velocity.
Calculator Between Two Dates
Find days, weeks, months, and years between dates.
Percentage Calculator
Calculate percentages, increases, and decreases quickly.
Disclaimer: This boat speed calculator is for general informational, educational, and planning use only. It does not replace seamanship, navigation training, manufacturer specifications, marine survey advice, professional mechanical diagnosis, local regulations, official charts, tide/current tables, weather forecasts, or safe boating judgment. Actual speed, fuel use, handling, and stopping distance can vary with current, tide, wind, waves, visibility, load, hull condition, propeller condition, engine health, trim, water depth, traffic, and operator decisions. Always operate within legal speed limits, posted no-wake zones, vessel capacity, engine ratings, and conditions you can safely handle.
Last updated: May 24, 2026