Definition
Buoyancy Factor — Buoyancy factor is the multiplier applied to a drill string's weight in air to find its buoyed weight when submerged in drilling fluid, calculated from the mud density and used to determine hook load and the neutral point.
Also known as: buoyancy factor, BF, buoyed weight factor
What the buoyancy factor is
The buoyancy factor (BF) accounts for the reduction in a drill string’s effective weight when it hangs in drilling fluid rather than air. A common form is:
BF = 1 − (mud density ÷ steel density), using about 65.5 lb/gal (or 489.5 lb/ft³) for steel.
For example, in 12.0 lb/gal mud the buoyancy factor is roughly 0.817, so the string weighs about 82% of its air weight while submerged. Multiplying the air weight of the string by the buoyancy factor gives the buoyed weight used to size the rig, calculate hook load, and locate the neutral point.
Why it matters
- Hook load — buoyed string weight, not air weight, sets the load on the rig and derrick.
- Neutral point — buoyancy changes the effective axial loads and the depth of the neutral point.
- Mud dependence — heavier mud gives a lower buoyancy factor and lighter buoyed weight.
Renting with accurate loads
Correct buoyancy calculations ensure the string and rig are matched to real downhole loads. We supply drill pipe and drill collars with documented weights for these calculations — contact us to plan your string.