Estimate under-keel clearance from charted depth, tide height, draft, and optional squat, wave allowance, and safety margin. Optional go/no-go against a required minimum UKC. Planning aid only — not a substitute for local notices or a pilot.
Use this when you have charted depth (or a sounding), tide height, and vessel draft and want under-keel clearance (UKC) with an optional safety margin flag.
Enter depth datum reading, tide height above that datum, draft, and optional minimum UKC you want to keep.
Not a substitute for local pilotage, siltation, or COLREGS — charts and eyes still win.
Under-keel clearance equals charted depth plus tide height, minus draft, optional squat, optional wave/swell allowance, and optional safety margin. Use one consistent unit set (feet or meters) throughout.
UKC = charted_depth + tide_height − draft − squat − wave − safety_margin
Planning aid only. Confirm chart datum, local tide predictions, and harbor guidance.
Example with the squat, wave, and safety allowances set to 0: charted depth 12 ft, tide +3 ft, draft 5.5 ft → water over bottom = 15 ft → UKC = 15 − 5.5 = 9.5 ft.
| Depth | Tide | Draft | UKC |
|---|---|---|---|
| 12 | 3 | 5.5 | 9.5 |
| 8 | 1 | 6.0 | 3.0 |
| 20 | 0 | 7.0 | 13.0 |
| 6 | 2 | 5.0 | 3.0 |
| 10 | -1 | 4.5 | 4.5 |
Rows: UKC = depth + tide − draft (same unit throughout).
Assumptions: depth and tide share a datum; draft is static (not squat); units match across fields.
Where it fails: squat at speed, heave in a seaway, uncharted silt, and transducer offsets if you typed a sounder reading without correcting to the keel.
Not a substitute: planning aid only — not pilotage, not a survey, not weather advice.
Extra sinkage underway in shallow water. Enter your allowance; this page does not invent a squat model.
Tide height must be on the same datum as the charted depth (e.g. MLLW). Mixing datums invalidates UKC.
If you enter a required minimum UKC, the page compares computed UKC to that threshold. Your call — not a navigation order.