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Boat Lift Water Depth Calculator

By Boat Lift Lab Updated 4 min read

Quick answer

A lift site has to pass two tests. At low water, depth must exceed boat draft plus the lowered bunk height plus clearance. At high water, the raised bunks must sit above the water surface plus a wave allowance, which sets the minimum lift travel.

On this page
  1. The two equations
  2. Measuring your inputs
  3. Worked example: lake lift with a 2 ft seasonal swing
  4. What the calculator cannot see
  5. Frequently asked questions
  6. Sources and further reading

Water depth checker

Boat and lift
in
Use the hull draft with outboard or sterndrive trimmed up.
in
For piling lifts, height of the lowered cradle above the bottom.
in
in
From the spec sheet; often 48 to 72 in on lake lifts, more on piling lifts.
Water levels at the lift spot
ft
ft
in
Wave, wake and surge allowance. More on open, windy or tidal water.

Results appear here as you type.

Checks the two conditions that decide whether a lift works: enough depth to float the boat on and off at low water, and enough travel to hold the hull clear at high water.

The two equations

Everything in this calculator comes from two inequalities. Measuring from the lake or sea bottom at the lift spot, in inches:

  • Low water test: low water depth ≥ boat draft + lowered bunk height + clearance
  • High water test: lowered bunk height + lift travel ≥ high water depth + wave allowance

The first test asks whether the boat can float on and off the cradle when the water is at its lowest. The second asks whether the raised boat sits above the water, waves included, when the water is at its highest. Rearranging the second gives the minimum travel your site needs: high water depth + wave allowance - lowered bunk height.

Notice the tension between them. A taller cradle (bunks higher off the bottom) helps the high water test but hurts the low water test. That is why sites with large water level swings run out of fixed-lift options, and why floating lifts exist.

Measuring your inputs

Boat draft

Use the draft of the hull with the outboard or sterndrive trimmed fully up, because that is how you drive on. Builders usually publish "draft, drive up" and "draft, drive down". If yours does not, measure from the waterline to the lowest point of the hull (often the keel near the stern) with the boat floating at its normal load.

Lowered bunk height

For a lake lift, measure from the bottom to the top of the bunks with the lift fully lowered and the feet settled. For a piling-mount lift, the lowest cradle position is set at installation, so use the height the installer can achieve. Remember that bunks need a few inches of water between them and the hull, which is what the clearance input covers.

Low and high water

Use real extremes, not averages. On tidal water, use local tide predictions for spring tides and add a surge allowance; on reservoirs, use the operator's published drawdown and flood pool levels; on natural lakes, use the lowest late-summer level you have seen and the spring high. Measure depth at the actual lift spot, not at the end of the dock. See water depth requirements and tidal installations.

Worked example: lake lift with a 2 ft seasonal swing

Worked example: 21 ft bowrider on a free-standing lift

Draft with the drive up: 20 in. Lowered bunk height: 14 in. Clearance: 6 in. Lift travel: 60 in. Water depth at the lift: 3.0 ft (36 in) in late August, 5.0 ft (60 in) in May. Wave allowance: 12 in.

  • Low water test: 20 + 14 + 6 = 40 in needed, 36 in available. Fails by 4 in.
  • High water test: 14 + 60 = 74 in reached, 60 + 12 = 72 in needed. Passes by 2 in.

The fix is not a taller lift. Moving the lift spot to where the late-summer depth is at least 40 in (and rechecking high water there) solves the low water problem. Moving out 6 ft in a gently sloping lake often adds 4 to 6 in of depth, but on most lakes it also adds depth at high water, so recheck both tests.

What the calculator cannot see

  • Soft bottoms. On silt, lift feet sink, which lowers the bunks over time and changes both tests. Use pads and recheck after the first season. See lake bottom conditions.
  • Uneven bottoms. Measure at all four leg positions; a slope means one end of the cradle sits higher.
  • Wakes. A busy channel can need far more than 12 in of allowance. Watch the dock on a summer Saturday before deciding.
  • Prop and drive clearance. Trimmed-up drives still need room behind the cradle crossmembers.

Frequently asked questions

How deep does the water need to be for a boat lift?

At the lowest water level you expect, depth must exceed your boat's draft with the drive up, plus the height of the bunks above the bottom when the lift is fully lowered, plus about 6 in of clearance. For many runabouts on lake lifts that works out to roughly 36 to 48 in, but measure your own boat and lift.

How high should a boat lift raise the boat above the water?

Enough that the hull clears the highest normal water plus waves and wakes. On sheltered lakes 12 in above high water is a common minimum; on open or tidal water 18 to 24 in or more is safer, and storm surge needs its own plan.

What if my water is too shallow for a boat lift?

Options include moving the lift farther out to deeper water, a low-profile cradle, lowering the cradle on a piling lift, dredging where permitted, or using the lift only above a minimum water level. Dredging almost always needs permits, so check first.

What is lift travel?

Lift travel is the vertical distance the cradle moves between fully lowered and fully raised. Your site needs at least the high water depth plus wave allowance minus the lowered bunk height. Manufacturers list travel in their specifications.

Sources and further reading

  • NOAA Tides and Currents, tide predictions, tidal datums (MLLW, MHHW) and station records: tidesandcurrents.noaa.gov.
  • U.S. Army Corps of Engineers and state agencies, reservoir operating levels and drawdown schedules.
  • Boat builders' published draft specifications (drive up and drive down).