ForHosting KIT · Travel & geography

Tide time for depth calculator

With high water at 5 and low water at 1.4, a target of 3.2 is reached 3 hours after high water on the falling tide, exactly half the 3.6 range. Time bar crossings, marina sills and drying moorings. Free in your browser.

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Worked example

If you enter

High water: 5
Low water: 1.4
Target height: 3.2

You get

Range: 3.6
Direction: Falling
Interval hours: 6
Fraction of range: 0.5

Skippers, harbour pilots, and coastal walkers often need a clock time, not just a height of tide. The almanac gives high water and low water; a bar, marina sill, or drying mooring asks a narrower question: when will the water reach the height I actually need? This tide time for depth calculator applies the rule of twelfths — the classic 1-2-3-3-2-1 split of the range over six tidal hours — and inverts it, so a target height between high and low water becomes hours and minutes from the starting stand. You may time a falling ebb from high water or a rising flood from low water, stretch the six tidal hours over a real interval such as 6.2 hours, and optionally attach a 24-hour clock.

The rule of twelfths and why skippers still use it

Between one high water and the next low water the sea does not fall at a constant rate. It changes slowly near the stands, then faster through mid-tide, in a curve that roughly follows a cosine. Long before electronic tide stations, navigators approximated that curve by splitting the range into twelve equal parts and assigning those twelfths to six hours: one twelfth in the first hour, two in the second, three in the third, three in the fourth, two in the fifth, and one in the sixth.

Cumulative from the starting stand the water has therefore moved 0, 1, 3, 6, 9, 11, and 12 twelfths of the range at hours zero through six. Half the range is gone at hour three — mid-tide — which is why a mid-height target is the easy mental case. The rule is an approximation, not a harmonic prediction.

It works best on a regular semi-diurnal coast with a single high and a single low, and it is weaker where the tide is diurnal, where a double high or a long stand flattens the curve, or where shallow-water distortion makes the flood much shorter than the ebb. Even so, it remains the method taught for Yachtmaster and coastal skipper exams, and it is the method a watchkeeper can still run in the cockpit when the plotter is busy.

This calculator encodes that same 1-2-3-3-2-1 table and inverts it, so you ask for a height and receive a time instead of walking the table hour by hour by hand.

Who needs a tide time for a target height

Bar-crossing and inlet briefs need a time window, not a pretty curve: if the bar is safe only above 1.8 metres on a 4.2-metre range, the skipper wants hours after low water on the flood and hours after high water on the ebb. Drying harbours and marina sills ask the same question in reverse — when will the tide lift the keel over the cill, and when will it leave the boat aground again.

Sailing schools and RYA-style quizzes need a deterministic oracle that marks half-tide at three hours and rejects a target above high water as invalid, not a chat answer that invents a cosine. Coastal walkers and photographers use the same invert to guess when a causeway or reef uncovers. Pin the rounded hours in tests, keep requests small, and you get reproducible rule-of-twelfths times for edge workers and briefing tools without standing up a harmonic tide station of your own.

What you can do with it

Bar and inlet crossing windows

Convert a minimum safe height on a published range into hours after low water on the flood and after high water on the ebb before you commit to a shallow bar.

Drying berth and marina sill times

Find when the tide will lift the keel over a cill or leave a drying mooring, using the almanac high and low and the height you actually need.

Skipper exams and briefing tools

Mark rule-of-twelfths items with a deterministic invert, or drop the same hours and hourly table into a passage brief or electronic log.

FAQ

What is the rule of twelfths?

It splits the tidal range into twelve parts over six hours: 1, 2, 3, 3, 2, then 1 twelfth per hour. This calculator inverts that 0-1-3-6-9-11-12 cumulative curve, interpolating inside each hour.

Can I time both a rising flood and a falling ebb?

Yes. Set direction to rising to count from low water, or falling (the default) to count from high water.

What if high and low water are not six hours apart?

Enter the real interval between the two stands, for example 6.2 hours. The six tidal hours of the rule are stretched evenly over that interval, so the time you get matches your almanac instead of a textbook half-cycle.

Where is the rule of twelfths less reliable?

It works best on a regular semi-diurnal coast with a single high and a single low. It is weaker with diurnal tides, a double high or a long stand, or shallow water that makes the flood much shorter than the ebb.

Can I get a clock time instead of hours after the stand?

Yes. Add the 24-hour clock time of the starting stand, high water on a falling tide or low water on a rising one, and the result also shows the clock time when the target height is reached.

Related tools

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/travel/tide-time-for-height

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/travel/tide-time-for-height \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"high_water":5,"low_water":1.4,"target_height":3.2}'

const res = await fetch("https://api.kit.forhosting.com/travel/tide-time-for-height", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "high_water": 5,
    "low_water": 1.4,
    "target_height": 3.2
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/travel/tide-time-for-height",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "high_water": 5,
    "low_water": 1.4,
    "target_height": 3.2
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/travel/tide-time-for-height", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"high_water":5,"low_water":1.4,"target_height":3.2}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"high_water":5,"low_water":1.4,"target_height":3.2}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/travel/tide-time-for-height", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "high_water": 5,
  "low_water": 1.4,
  "target_height": 3.2
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "travel.tide_time_for_height",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Limits

max_interval_hours 18

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Technical notes

Heights outside the published range fail with invalid_input and are not billed. Successful calls cost the published $0.002 with no model tokens and no outbound network.

How a target height becomes hours, minutes, and a clock

The range is high water minus low water and must be positive. Success also returns hours and minutes from both stands, height above low water, remaining rise and fall, a short message, and an hourly table of heights so you can check the 1-2-3-3-2-1 steps. Send high_water and low_water in any consistent unit — metres or feet — and a target_height that lies on or between those stands. The fraction of the range already changed is (high_water minus target) when you time a falling tide from high water, or (target minus low_water) when you time a rising tide from low water. That fraction is inverted on the cumulative twelfths knots with linear interpolation inside each hour: a target one twelfth of the way through the range is exactly one tidal hour; a target two twelfths of the way sits halfway through the second hour, and so on. interval_hours defaults to six, the textbook half-cycle; if your almanac shows six hours twelve minutes between stands, send 6.2 and the six tidal hours stretch uniformly. direction is falling or rising; omit it and the calculator times the ebb from high water, the usual “when will it drop to this height” question. Optional start_time is a 24-hour HH:MM clock of that starting stand; the response then includes clock_time and a next_day flag when the offset crosses midnight. Missing numbers, a non-positive range, a target outside the stands, an interval outside 0.25–18 hours, a bad direction, or a malformed clock fail with invalid_input and are not billed.

Where a tide-time-for-height API earns its keep

Passage-planning software and electronic logbooks can call this endpoint instead of embedding another copy of the twelfths table that will drift from the exam book.

Technical questions

How do I find when the tide reaches a target height?

Send high_water, low_water, and target_height in the same unit. The API inverts the rule of twelfths and returns hours and minutes from the starting stand (high water if falling, low water if rising).

What if my target is outside high and low water?

The task fails with invalid_input. A height above high water or below low water cannot occur on that tide, so the request is rejected and is not billed.

What does the tide time for depth calculator cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.

Read the full KIT documentation →

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