Tidal Current Speed Calculator
A reversing tidal stream is slack at the turn, builds to a maximum in the middle of its run, then eases back to slack. The navigator's Fifty-Ninety rule sketches that pattern: as fractions of the maximum, the hourly speeds run about 50, 90, 100, 100, 90, and 50 percent across the six hours. Enter the maximum current rate and this calculator estimates the speed in each hour after slack. Always confirm with official tidal current data before navigating.
Fifty-Ninety rule
Hour 1: 0.50 * max
Hour 2: 0.90 * max
Hour 3: 1.00 * max
Hour 4: 1.00 * max
Hour 5: 0.90 * max
Hour 6: 0.50 * max
The fractions approximate a sinusoidal build and ease of a reversing stream between slack waters.
Worked example
- Maximum current 3 knots.
- Hour 1 and hour 6: 0.50 * 3 = 1.50 knots.
- Hour 2 and hour 5: 0.90 * 3 = 2.70 knots.
- Hours 3 and 4 (peak): 1.00 * 3 = 3.00 knots.
Tidal current: frequently asked questions
What is the Fifty-Ninety rule?
The Fifty-Ninety rule estimates how a reversing tidal current builds and eases over the roughly 6 hours between slack and the next slack. As a fraction of the maximum current, the hourly speeds are about 50, 90, 100, 100, 90, and 50 percent. It is a navigator's rule of thumb for streams that follow a roughly sinusoidal pattern.
When does this rule apply?
It applies to a reversing (rectilinear) tidal stream that runs about 6 hours one way, then 6 hours the other, with slack water in between. It assumes a smooth, near-sinusoidal build and ease. Local channels, rivers, and rotary currents can differ markedly, so always check official tidal stream data.
How do I use the maximum current figure?
Take the maximum rate for the stream from a tidal stream atlas or current table, enter it, and the calculator multiplies it by the hourly percentages to estimate the speed in each hour after slack. The peak occurs in the middle two hours.
Is this as accurate as published current tables?
No. The Fifty-Ninety rule is a quick estimate for planning. For navigation you should use official tidal current predictions, such as NOAA tidal current tables, which account for local timing and rates. This tool helps you sketch the pattern between slack waters.
Official sources
- NOAA: tidal current predictions and tables.
- NOAA Office of Coast Survey: nautical charts and currents.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.