Tidal Current Speed Calculator
Estimate tidal current speed at any point in the tidal cycle using the 50/90 rule.
Enter max current speed and time before or after slack water in hours.
Tidal current does not flow at a constant rate — it accelerates from slack water, reaches maximum flow at mid-tide, then slows again. The 50/90 rule describes this pattern.
The 50/90 rule (6-hour tidal cycle):
- Hour 1 (after slack): 50% of maximum current
- Hour 2: 90% of maximum current
- Hour 3: 100% of maximum current (peak flow)
- Hour 4: 90% of maximum current
- Hour 5: 50% of maximum current
- Hour 6: Approaching slack water (near 0%)
More accurate: sinusoidal approximation Current at time t: V(t) = V_max × sin(π × t / T)
Where t is hours after slack and T is the half-cycle duration (typically 6 hours).
Why this matters for sailors:
- Plan passages to use favorable current or avoid adverse current
- At 2 knots of adverse current, a boat making 5 knots over the water makes only 3 knots over the ground
- Spring tides (full/new moon) produce stronger currents than neap tides
- Tidal currents in narrows can reach 8–12 knots (e.g., Skookumchuck, Saltstraumen), making them impassable for most small boats at peak flow
Tip: UK Nautical Almanac and NOAA tide current tables provide predictions at specific locations. This calculator gives a general approximation for planning purposes.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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