Tidal Current Speed Calculator

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%)

The underlying curve 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. This calculator fixes T at 6 hours, which is the standard assumption for a semi-diurnal coast.

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
• The famous narrows run far harder than most sailors expect. Saltstraumen in Norway reaches about 20 knots and Skookumchuck Narrows in British Columbia about 16, which is impassable for anything without serious power, and both are worth timing to the minute rather than the hour

The two numbers disagree, and that is fine

Run the sine and the 50/90 rule side by side and they part company in the second hour: the sine gives 86.6% of maximum where the rule says 90%. The rule is the approximation, not the sine. Somebody rounded sin(60°) up to a number you can do in your head on a wet deck, and 3.4 percentage points is a fair price for that. The calculator shows both so you can see the size of the gap.

Slack water is not the same as high water

This is the assumption that catches people out. In a standing-wave estuary the stream does turn near high and low water, and the rule works as written. But in a progressive tidal wave, or through a narrow entrance draining a large basin, slack can fall an hour or more away from the tidal height turning point, sometimes closer to mid-tide. The hours you feed this calculator have to be counted from actual slack, taken from a tidal stream atlas or the chart diamonds, not from the tide table.

Not to be confused with the rule of twelfths

The twelfths rule (1, 2, 3, 3, 2, 1 over six hours) describes tidal height, how far the water rises each hour. This page is about rate, how fast the stream is running. They are different curves answering different questions, and reading the twelfths as a stream rule makes the first hour after slack look far calmer than 50% of maximum.

The number worth carrying off this page is the one the table makes least obvious: an hour either side of peak the stream is still at 90%. It takes two hours to fall to half. For anyone timing a crossing under their own power, the tidal current effect calculator for kayakers works out what a given stream does to speed over ground and to the heading you need to hold.

Tip: UK Nautical Almanac and NOAA tidal current tables provide predictions at specific locations. This calculator gives a general approximation for planning purposes.