Compare a fractional rule-of-twelfths interpolation between entered same-station tide events with an entered official height.
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Compare a fractional approximation between entered official same-station tide events with an entered official prediction or observation without making a tide/navigation claim.
Keep station, datum, date, time zone and actual event interval aligned.
Enter the official low/following-high events, elapsed time and an official comparison height.
Sees the interpolation difference without treating it as an official prediction.
Avoid whole-hour rounding and a fixed six-hour assumption.
Use the actual event interval and inspect the fractional normalized phase.
Can reproduce the approximation and compare it with the station product.
Keep water-level arithmetic below current official navigation evidence.
Use official predictions/observations, charts/notices/weather and vessel draft/squat/heel margins externally.
Never navigates or claims clearance from the interpolation.
Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.
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The entered elapsed time is normalized across the actual entered low-to-high interval, then linearly interpolated between cumulative 0, 1, 3, 6, 9, 11 and 12 twelfths rather than rounded to a whole hour.
Low, following high and comparison heights must share station, datum, units and event date/time-zone context. The tool does not retrieve or generate official predictions.
NOAA harmonic predictions use station-specific constituents. This approximation is compared with an entered official value and cannot determine observed water level, current, charted depth or under-keel clearance.
Updated: August 2026
Reproduce a rule-of-twelfths worksheet from one official low event, its following high event and their actual time interval.
Evaluate a half-hour or other fractional point continuously instead of silently rounding it to a whole twelfths hour.
Compare the approximate interpolated height with a station/date/time/datum-specific official prediction or observation entered from NOAA.
Use the same official station, datum, unit and time-zone basis for the low event, following high event and comparison height; verify the exact date and event order.
Use current official predictions/observations, charts, notices and weather plus vessel draft, squat, heel and conservative margins. This approximation cannot establish depth or under-keel clearance.
Interpolate one entered rising-tide event interval and compare it with an entered official station value. This is a transparent approximation—not an official prediction or navigation tool.