Keel Clearance Calculator (Will I Float?)
Charted depth + tide height − draft − safety margin: the under-keel arithmetic, with chart-datum logic explained so the sounding makes sense.
Chart datum is set near the LOWEST astronomical tides precisely so charted depths are the pessimistic floor — the tide table's height ADDS to the chart. Negative table entries (below-datum lows) exist and subtract; spring lows are when they appear.
Formula
⚠️ For planning and education only — verify with official charts, tide tables and your vessel's documentation. Not for navigation.
Charted depth + tide height − draft − safety margin: the under-keel arithmetic, with chart-datum logic explained so the sounding makes sense.
About Keel Clearance Calculator (Will I Float?)
“Is there enough water?” decomposes into one addition and one subtraction — once you understand the datum trick: charted depths are referenced to roughly the lowest tides, so the table's tide height ADDS to the chart's number. This calculator runs the under-keel arithmetic with your demanded margin, flags the squeeze zone, and computes the minimum tide that opens a too-shallow gate — the planning number for every drying-harbor entrance.
How to use Keel Clearance Calculator (Will I Float?)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula UKC = charted depth + tide height − draft; pass when UKC ≥ margin substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Keel Clearance Calculator (Will I Float?)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula UKC = charted depth + tide height − draft; pass when UKC ≥ margin with sources cited on the page
- ✓Chart datum is set near the LOWEST astronomical tides precisely so charted depths are the pessimistic floor — the tide table's height ADDS to the chart. Negative table entries (below-datum lows) exist and subtract; spring lows are when they appear.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
What exactly is chart datum?+
The depth reference surface, chosen pessimistically low — MLLW (mean lower low water) on US charts, LAT (lowest astronomical tide) on most international ones — so that the charted number is close to the least water you'd ever find. Tide heights in tables are measured UP from this same surface: chart + tide = actual water, by construction. Negative tide entries mean even-lower-than-datum spring lows: the rare days the chart itself is optimistic.
What belongs in the safety margin?+
Everything the two clean numbers don't know: swell (half the wave height comes OFF your clearance in troughs), squat (boats settle underway in shallow water — a knot-dependent few inches to feet), chart sounding age and shifting bottoms (sandbars move between surveys), tide-table error (±0.5 ft routinely with weather), and your draft's honesty (loaded? listing?). Two feet is the calm-water floor; swell multiplies it.
How do I plan a transit that fails at the current tide?+
Invert the question, as this tool's last output does: required tide = draft + margin − charted depth. Then consult the curve (rule of twelfths or the port's tables) for when that height arrives — and prefer transiting on a RISING tide, the seaman's insurance: touch on the flood and the next half hour lifts you off; touch on the ebb and you'll be photographing the keel for six hours.
Why did I find less water than chart + tide promised?+
The usual suspects, in order: weather tide (high pressure and offshore winds depress real water below predictions — a steady barometer at 1035 hPa is ~8 inches of missing tide), shoaling since the survey (check the chart's source diagram for sounding dates), squat at speed, and waves. The arithmetic is exact; its inputs are estimates — which is precisely what the margin input is for.
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