Speed Over Ground with Current Calculator

Enter boat speed and direction, then current speed and the direction toward which it flows. The calculator resolves both vectors into east and north components, adds them, and returns speed over ground, course over ground, and useful component values. It supports knots, kilometres per hour, miles per hour, and metres per second for passage planning, training, and transparent navigation checks.

Enter through-water motion and current correctly

Start with the boat speed through the water, not a GPS speed that already includes current. Pair it with the direction in which the boat is moving through the water. Both direction fields use true bearings measured clockwise from north: north is 0 degrees, east is 90, south is 180, and west is 270. Current drift is the current's speed, while current set is the direction toward which the water flows. That last distinction matters because some weather descriptions name the direction a wind comes from, but a marine current set normally describes where the water is going. Use the same speed unit for boat speed and drift; the selected unit applies to both and to the primary speed-over-ground result. A bearing of 360 degrees is accepted as equivalent to north. If your source gives magnetic headings, apply the appropriate variation before using this true-bearing calculator. Also distinguish heading from actual through-water course when leeway is material: the vector calculation can only represent the direction you enter, so account for leeway separately before treating the result as a navigation prediction.

How speed and course over ground are calculated

The calculator treats each motion as a two-dimensional velocity vector. For a true bearing, its east component is speed multiplied by the sine of the bearing, and its north component is speed multiplied by the cosine. It computes those components for the boat and current, then adds east to east and north to north. The magnitude of the combined vector, found with the Pythagorean relationship, is speed over ground. Course over ground is derived with a two-argument arctangent using east and north, then normalized to a bearing from 0 up to but not including 360 degrees. This method handles following, opposing, and cross currents without special cases. The response includes resultant east and north velocity in the selected unit, speed over ground in that unit and in knots, and the difference between speed over ground and boat speed. That difference is a resultant speed effect, not simply the current drift, because a cross-current can change direction strongly while producing a smaller change in overall speed. Results are rounded to six decimal places so repeated API and browser calculations remain stable.

Use the result as a planning calculation

Use speed over ground to estimate progress along the resultant track and course over ground to see where the combined motion points. A following current generally raises speed over ground, an opposing current lowers it, and a cross current shifts course while also changing the resultant speed. The returned component values are useful for checking the geometry or feeding a later calculation. However, this is a single-state vector model: it assumes boat velocity and current remain constant over the interval being considered. Real currents vary with position, depth, tide, river discharge, and time, while steering, waves, leeway, and manoeuvres alter the boat vector. The result therefore supports route comparison, instruction, and short-interval estimates rather than replacing current charts, local notices, GPS observations, or a qualified navigator. For passage planning, divide the route into legs when conditions change and use conservative inputs. Recalculate underway with current observations, maintain a proper position fix, and follow applicable navigation rules. If the two vectors cancel exactly, the boat has zero speed over ground and no meaningful course over ground, so the course field is deliberately omitted.