Outboard Size Calculator
Enter the boat's loaded weight, how you use it, the transom height and the maximum horsepower on the capacity plate. The calculator suggests an outboard size, the shaft length and the likely top speed.
- Horsepower from weight
- Shaft from transom
- Top speed estimate
Loaded weight is hull, engine, fuel, people and gear together. Transom height is measured from the top of the transom to the bottom of the hull.
Outboards are sized by the weight they push. A fishing boat needs about one horsepower for every 35 lb of loaded weight. Towing a skier takes more, about one for every 25 lb. The boat's capacity plate sets the legal limit.
Outboard size chart
Fishing use. Search for a weight.
| Loaded weight (lb) | Outboard (hp) | Speed (mph) |
|---|---|---|
| 500 | 15 | 31 |
| 1000 | 30 | 31 |
| 1500 | 50 | 33 |
| 2000 | 60 | 31 |
| 3000 | 90 | 31 |
| 4000 | 115 | 30 |
| 5000 | 150 | 31 |
Speeds are rough, for a planing hull in calm water.
Weight and horsepower
Fishing use. Slide the loaded weight.
The boat grows with weight and the engine with its horsepower.
How to size an outboard
Weight, use, plate. Step through it.
Loaded weight
Everything aboard.
A motor that is too small runs flat out all the time, using more fuel than a larger motor at part throttle.
Two-stroke, four-stroke or electric
Outboard types. Compare them.
Quiet and economical.
Light and punchy.
Silent, no fumes.
Electric outboards suit small boats and lakes with engine bans, but range is limited by the battery.
Before buying
Engine checks. Tick each item.
Tick each item as you go.
The propeller pitch should let the engine reach its rated rpm at full throttle with a normal load.
What you get
Outboard sizing. Each tile explains one result.
Standard hp.
From weight.
Short to ultra.
Rough top speed.
Capacity plate.
Shaft length matters: too short and the propeller sucks air, too long and it drags.
Frequently Asked Questions
No question matches that search.
What size outboard do I need?
About one horsepower for every 35 lb of loaded weight for fishing, or every 25 lb for watersports.
How do I find my boat's maximum horsepower?
Look at the capacity plate near the helm or on the transom.
What shaft length do I need?
Short (15 in) for transoms up to about 17 in, long (20 in) up to 22 in, extra long (25 in) above that.
Is a bigger outboard more economical?
Often a little, because it runs at a lower throttle setting, but it costs more and weighs more.
How fast will my boat go?
Roughly 180 divided by the square root of pounds per horsepower, in mph, for a planing hull.
Can I put too big an outboard on a boat?
Not legally above the capacity plate, and it can make the boat unsafe.
Do I need more power to pull a skier?
Yes, about 40% more than for gentle cruising.
Power matched to your boat
Start from the loaded weight, choose the power for how you use the boat and stay within the capacity plate. Then match the shaft to the transom.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.