Calculator

Drill Pipe Lag & Volume

Pump output, string and annular volumes, and lag strokes and lag time, both down to the bit and back up to surface.

⚙ Pump
🔩 Drill String (internal)
⭕ Annulus
Pump Output—
Pump Rate—
String Capacity Volume (internal)—
String Steel Displacement—
Total Annular Volume—
Lag Strokes Down (to bit)—
Lag Strokes Up (bottoms-up)—
Lag Time Down—
Lag Time Up (bottoms-up)—
Reference only. Measure pump output and volumetric efficiency against your actual equipment rather than trusting nameplate figures — a worn pump quietly throws off every number below it.

The Formulas Used

Pump Output — Triplex
Output (bbl/stk) = 0.000243 × Liner² (in) × Stroke (in) × Efficiency
Pump Output — Duplex
Output (bbl/stk) = 0.000162 × Stroke × (2 × Liner² − Rod²) × Efficiency
Capacity and Displacement
Capacity (bbl/ft) = ID² ÷ 1029.4
Displacement (bbl/ft) = (OD² − ID²) ÷ 1029.4
Annular capacity uses the same shape: (hole ID² − pipe OD²) ÷ 1029.4.
Lag Strokes and Time
Strokes = Volume (bbl) ÷ Pump Output (bbl/stk)
Time (min) = Strokes ÷ SPM
Down uses drill string capacity. Up (bottoms-up) uses annular volume.

These are the same capacity and displacement formulas behind the kill sheet calculator's strokes-to-bit and bottoms-up figures — this page just runs them for routine lag correlation rather than a kill. If you need a running joint-by-joint depth tally rather than a one-shot volume, see the drill pipe tally calculator. For the constants written out with worked numbers, see the drilling formulas cheat sheet.

Frequently Asked Questions

What is lag time and why does it matter?

Lag time is how long it takes a sample of fluid, cuttings, or gas at the bit to reach surface after it enters the annulus. Without it, a gas show or cuttings change at surface can't be tied back to the depth that produced it — every mud logging correlation depends on getting lag right first.

Why does pump type change the output-per-stroke formula?

A triplex pump displaces fluid on one side of the piston per stroke (single-acting), so its output depends only on the liner area and stroke length. A duplex pump displaces on both sides (double-acting), but the rod takes up cross-sectional area on one side, so its formula subtracts the rod area — leaving it out overstates duplex output.

What is the difference between capacity and displacement?

Capacity is the volume inside the pipe (uses ID) — what fills it and what a pump has to move to fill it. Displacement is the volume of steel the pipe itself takes up (uses OD minus ID) — what the pipe pushes out of the way when it's run into a full hole. Both come from the same formula shape, just different diameters.

Why measure volumetric efficiency instead of assuming 100%?

Piston and valve wear let a pump slip fluid backward on part of every stroke, so real output is always somewhat below the theoretical figure — typically 95-100% freshly serviced, lower as pumps wear. Since every strokes-based number on this page scales off pump output, an assumed efficiency quietly compounds into every downstream figure.