iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
The 50/90 rule is a rough way to estimate tidal-current speed between slack water and peak flow: at one-third of the slack-to-peak interval, the current is about 50% of its peak; at two-thirds, about 90%; and at the interval’s end, it reaches peak. The pattern reverses as the current slows toward slack. It estimates water moving horizontally—not the tide’s height—and should not replace local current predictions when planning a boat trip.
How the 50/90 rule works
Start with the time from slack water—the period of little horizontal movement—to peak current. Divide that interval into three equal parts. The rule estimates current speed at about 50% of peak after the first part, 90% after the second, and 100% at peak. As the flow weakens, the estimates reverse.
The shorthand sequence is 0/50/90/100/90/50/0: slack, increasing flow, peak, decreasing flow, then slack again. These percentages are estimates in a navigation rule of thumb, not results from a published accuracy study. Starpath’s navigation material describes the rule; use thirds of the actual slack-to-peak interval rather than assuming fixed hourly steps.
Quick wins for a faster PC:
Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Example using a three-hour interval
If the interval from slack to peak is three hours, the estimate is 50% of peak after one hour, 90% after two hours, and peak after three hours. The reverse sequence applies after peak. The clock times in this example illustrate the calculation; they are not a tide prediction.
#1 Best Overall
- Comprehensive Guide: Complete reference to essential boating safety rules and regulations with easy-to-understand instructions for emergency situations and safety procedures while on the water
- 100% Waterproof Construction: Built with high-quality, weather-resistant materials to withstand the rigors of outdoor use, ensuring long-lasting reliability on every trip
- Portable & Lightweight Design: Convenient, travel-friendly size fits in any equipment bag or can be stowed away on board, allowing you to have it on hand at a moments notice
- Suitable for All Boaters: Designed for beginners, seasoned sailors, kids and scouts, and any outdoor enthusiasts looking to master the rules of good seamanship
- Double-Sided Reference Chart: Features nautical flag signals, mayday signals, fog signals, sound signal definitions, overtaking procedures, emergency firefighting, and life-saving first aid information for both Intercoastal and International waters
What it estimates—and what it does not
The rule estimates tidal-current speed, the horizontal movement of water. It does not estimate tide height, the vertical rise and fall of the water. NOAA explains the distinction between tides and tidal currents in its tides overview.
Do not confuse it with the Rule of Twelfths, which is a rough method for estimating changes in water height between high and low water. The two rules estimate different things. Neither is a substitute for a location-specific prediction.
Why the estimate has limits for boating
Slack current may not coincide with high or low water
The time of slack current and the time of high or low tide do not necessarily match. The relationship varies by location, so a high- or low-water time cannot automatically be used as the slack point for the 50/90 calculation. NOAA describes the relationship between tides and currents as a general rule of thumb, not a universal timing rule.
Free tools Windows power users keep installed
One-click scans. No signup required.
Conditions can differ from astronomical predictions
NOAA’s predictions cover astronomical tides—water-level changes driven by the Moon, Sun, and their relative motions. They do not capture short-term changes caused by wind, rain, freshwater runoff, and other meteorological effects. Observed water levels and currents can therefore differ from the predicted pattern.
The percentages are not a safety guarantee
The 50% and 90% figures are the rule’s approximate milestones; the sources do not establish a measured accuracy rate. Use the rule only as a mental estimate, not as proof that a route, channel, or crossing is safe.
How to use it when planning a trip
- Find local current information. Look up predictions for the specific location and the relevant date, including current speed and direction. NOAA provides tide and current prediction services.
- Identify the local slack-to-peak interval. Do not infer slack from high or low tide unless local information supports that timing.
- Divide the interval into thirds. Use the 50%, 90%, and 100% estimates at the first, second, and third divisions.
- Use the prediction for the decision. Consider current direction and speed along with tide height and timing, especially in shallow ports, intracoastal waterways, and estuaries. The estimate does not account for changing weather or runoff.
NOAA distinguishes tide tables, which give times and heights, from current tables, which give times and speeds for flood, ebb, and slack. Its annual printed tide and current tables were discontinued after 2020; its online services provide predictions. See NOAA’s Tides & Currents FAQ for prediction context.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.50/90 rule versus tide-height rules and predictions
| Method or information | What it describes | Basis | Best use |
|---|---|---|---|
| 50/90 rule | Approximate tidal-current speed | Thirds of the slack-to-peak interval | A rough mental estimate of how flow builds and eases |
| Rule of Twelfths | Approximate change in water height | Fractions of the high-to-low tide interval | A rough estimate of vertical water-level change |
| Local current prediction | Predicted current speed and direction at a location | Location-specific prediction | Trip timing and route planning, alongside tide-height information |
NOAA’s tide information explains why current behavior and water height are distinct, while its prediction services provide location-specific information for navigation decisions.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
