Tidal Current Velocity Calculator
A tidal current builds from zero at slack water to a maximum near mid-cycle, then falls back to zero at the next slack. The 50-90-100 rule is a quick way to estimate the speed at any hour of that cycle: about half the maximum one hour after slack, ninety percent at two hours, and the full maximum at three hours, then symmetrically back down. Enter the maximum current rate for your location, taken from NOAA Tidal Current Tables, and the hour since slack water, and this calculator returns the estimated current speed.
50-90-100 rule
Hour 1: speed = 0.50 x maximum
Hour 2: speed = 0.90 x maximum
Hour 3: speed = 1.00 x maximum
Hour 4: speed = 0.90 x maximum
Hour 5: speed = 0.50 x maximum
Hour 6: speed = 0.00 (slack)
The factor is taken from the hour elapsed since slack water and applied to your stated maximum current rate. The cycle is symmetric about the mid-cycle maximum at hour 3.
Tidal current notes
- Take the maximum rate from NOAA Tidal Current Tables or a current diamond for your position.
- Spring tides give larger maxima than neap tides; use the right value for the day.
- The rule assumes a single, roughly sinusoidal current cycle.
- Local topography, rivers, and eddies can break the pattern; verify with official predictions.
- Slack water is timed separately from high and low tide at many locations.
Tidal current velocity: frequently asked questions
What is the rule of thirds for tidal currents?
The 50-90-100 rule (sometimes called the rule of thirds) estimates current speed during a six-hour tidal cycle: roughly 50 percent of the maximum rate one hour after slack, 90 percent at two hours, and 100 percent at three hours (mid-cycle). It then falls back symmetrically to slack water.
How is current speed estimated from the maximum rate?
Multiply the maximum (mid-cycle) current rate by the fraction for the hour since slack water: 50 percent at hour 1, 90 percent at hour 2, 100 percent at hour 3, 90 percent at hour 4, 50 percent at hour 5, and zero at hour 6. This calculator applies that factor to your stated maximum rate.
Where do I get the maximum current rate?
The maximum spring and neap current rates are published in NOAA Tidal Current Tables and on tidal current charts and diamonds. Enter the maximum rate for your location and tidal state. Because it is location-specific, the maximum rate is a user input here rather than a fixed figure.
How accurate is the 50-90-100 rule?
It is an approximation for places where the current follows a roughly sinusoidal cycle around a single maximum. It works well in open channels but poorly where local topography, river flow, or multiple constituents distort the cycle. Always cross-check with official current predictions for your area.
What is slack water?
Slack water is the brief period when a tidal current is at or near zero as it reverses between flood and ebb. The 50-90-100 rule measures elapsed time from slack water, so the speed starts at zero, rises to the mid-cycle maximum, then falls back to zero at the next slack.
Official sources
- NOAA Tides and Currents: Tides and Currents.
- NOAA Office of Coast Survey: Nautical Charts.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.