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The 50/90 rule is a rough way to estimate tidal-current speed between slack water and peak flow: the current is about 50% of peak one-third of the way through the interval, about 90% at two-thirds, and at peak at the end. It estimates horizontal water movement—not tide height—and is not a substitute for local current predictions when planning a route.
What the 50/90 rule estimates
The rule estimates how quickly water is moving as a tidal current builds from slack water toward peak flow, then weakens again. Its shorthand sequence over a full slack-to-slack cycle is 0/50/90/100/90/50/0: slack, rising current, peak, and falling current.
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NOAA distinguishes tides—the vertical rise and fall of water—from tidal currents, which are the horizontal movement of water. Flood current generally flows into bays, harbors, and estuaries as water rises; ebb current generally flows outward as water falls. Slack water is a period of little horizontal movement. NOAA’s tides and currents FAQ explains these terms.
How to apply the rule
- Find the local time from slack water to peak current using a location-specific current prediction or observation.
- Divide that slack-to-peak interval into three equal parts.
- At the first division, estimate the current at about 50% of peak speed; at the second, about 90%; at the end, peak speed.
- After peak, use the reverse progression: about 90%, then 50%, then slack.
For example, if the slack-to-peak interval is three hours, the rough estimates fall at one hour (50%), two hours (90%), and three hours (100%). Those are fractions of peak current speed, not fixed speeds in knots. The rule is not a prediction of the exact current at a particular time.
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Why the familiar hourly sequence is only an illustration
A six-hour slack-to-slack example can show the sequence at successive hourly marks: 0%, 50%, 90%, 100%, 90%, 50%, 0%. That is a convenient illustration, not a universal tidal schedule. The one-hour steps make sense only when slack-to-peak takes about three hours; if the local interval differs, divide its actual duration into thirds instead. Starpath’s navigation explanation presents the 50/90 method and cautions that hourly intervals are approximate: Inland and Coastal Navigation, 2nd edition.
How it differs from the Rule of Twelfths
The 50/90 rule estimates current speed between slack and peak flow. The Rule of Twelfths estimates changes in water height between high and low water. They address different quantities, so one cannot be used as a substitute for the other.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What boaters should check before relying on an estimate
- Use local current information. NOAA provides location-specific tide and current predictions. For navigation in shallow ports, intracoastal waterways, and estuaries, tide time and height matter alongside current speed and direction. See NOAA Tides & Currents and its tides and currents navigation guidance.
- Do not equate slack with high or low tide. The timing relationship is only a rule of thumb and varies by location; slack current may not coincide with high or low water.
- Allow for weather and runoff. Astronomical predictions do not capture short-term water-level changes caused by wind, rain, freshwater runoff, and other meteorological effects. Conditions on the water can differ from the prediction.
- Use the right prediction type. Tide tables give water-height times and values; current tables give the timing and speeds of flood, ebb, and slack. NOAA says annual printed tide tables ceased after 2020 and directs users to online prediction services; consult the relevant online service for current local information.
The 50% and 90% figures are the rule’s approximate estimates, not the results of a published accuracy study. Treat them as a mental guide to how flow may build or ease, not a guarantee of safe passage or a substitute for assessing local conditions.
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