Tidal Current Speed Calculator
Estimate tidal current speed at any time using the 50/90 rule.
Enter maximum current and hours before/after slack water.
Tidal Current Speed
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.