Floating Draft Calculator (Barge, Pontoon, Flat-Bottom Boat)
How deep a flat-bottomed box, pontoon or barge sits in the water for a given load, the freeboard left, and the most it can carry before the water reaches the deck.
Results
The dashed line marks the value you entered.
What this tool does
A floating hull sinks until the water it pushes aside weighs as much as it does — Archimedes' principle — and for a flat bottom with vertical sides that gives draft = mass ÷ (water density × length × width). Only the footprint matters, not how tall the sides are: double the footprint and the draft halves. Sea water is about 2.5 % denser than fresh, so the same boat floats slightly higher at sea. Reaching the deck edge is the sinking limit, but boats swamp or capsize well before it, so keep generous freeboard; this calculator ignores waves, stability and where the load sits.
Formula
draft = m ÷ (ρ × L × B) · freeboard = depth − draft · m_max = ρ × L × B × depth
Variables
| Symbol | Meaning | Unit |
|---|---|---|
m | Total mass afloat (hull plus load) | kg |
l | Length | m |
w | Width | m |
hd | Height of the sides (bottom to deck edge) | m |
rw | Density of the water (fresh about 1000, sea about 1025) | kg/m³ |
D | Draft (how deep it sits in the water) | m |
FB | Freeboard (negative means it sinks) | m |
CAP | Most mass it can carry before the water reaches the deck edge | kg |
EX | Extra load still possible (negative means overloaded) | kg |
Worked example
- Total mass afloat (hull plus load)500 kg
- Length3 m
- Width1.5 m
- Height of the sides (bottom to deck edge)0.5 m
- Density of the water (fresh about 1000, sea about 1025)1000 kg/m³
- Draft (how deep it sits in the water)0.111 m
- Freeboard (negative means it sinks)0.389 m
- Most mass it can carry before the water reaches the deck edge2250 kg
- Extra load still possible (negative means overloaded)1750 kg
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.