Check your laser level or automatic level calibration using the two-peg test. Enter four rod readings from two instrument setups and this calculator computes the collimation error in feet per 100 feet, tells you whether the instrument passes your tolerance, and provides correction instructions. Run this check before any critical elevation or grade work.
A level instrument with a 0.05 ft/100ft error seems small — but on a 200-foot shot from instrument to rod, that's a 0.10-foot (1.2 inch) error in your elevation reading. Over a day of work on a flat grade project, this can accumulate into significant out-of-tolerance conditions that aren't discovered until the inspector arrives. For a pipe laser or tight-tolerance flatwork project, even 0.02 ft/100ft error can cause failures.
The two-peg test eliminates the error through symmetry. By taking balanced near and far shots from both ends of the interval, random errors cancel and only the systematic collimation error remains — the constant tilt of the instrument's line of sight relative to horizontal.
| Instrument Type | Recommended Interval | Key Tolerance |
|---|---|---|
| Rotating laser (general construction) | Monthly or before major jobs | ±0.05 ft/100ft |
| Rotating laser (precision grade) | Weekly or bi-weekly | ±0.02 ft/100ft |
| Automatic level (builder's level) | Monthly | ±0.05 ft/100ft |
| Automatic level (engineer's level) | Before every critical job | ±0.02 ft/100ft |
| Digital level | Per manufacturer spec | ±0.005 ft/100ft |
| Pipe laser | Before each major pipe run | ±0.02 ft/100ft |
The two-peg test (also called a peg test or bubble tube test) is a field procedure to check the accuracy of a level instrument. Two pegs are set about 100–200 feet apart. Rod readings are taken at both pegs from each of two instrument setups. The four readings reveal any systematic collimation error — the angle between the line of sight and a truly horizontal plane.
Best practice: before every critical job, after any impact or rough transport, when readings seem inconsistent, and at the start of each season. Laser levels and automatic levels should be checked monthly if used daily. Topcon recommends monthly checks; Leica specifies quarterly at minimum. Consult your instrument manual for manufacturer-specific intervals.
Common field tolerances: general construction 0.05 ft/100ft (6mm/30m), road and site grading 0.02 ft/100ft (2mm/30m), precise leveling 0.005 ft/100ft (0.5mm/30m). Use the tighter tolerance for precision grade work and the general construction tolerance for most site grading and utility work. New instruments typically have tighter factory-spec tolerances.
Many automatic levels and rotating laser levels have field-adjustable screws for the compensator bubble and cross-hair position. After identifying the error with the two-peg test, set up at the midpoint between pegs, calculate the true reading, and adjust until the instrument reads correctly. Re-run the two-peg test to confirm. If the error cannot be eliminated in the field, the instrument needs service.
Yes. Extreme temperature changes — hot sun heating one side of the instrument body, or cold morning startup in warm storage conditions — cause temporary calibration shifts due to differential thermal expansion. Best practice: shade the instrument during setup, allow 15 minutes for thermal equilibration, and re-check calibration if temperature changes more than 20°F during the workday on precision projects.
Calculate collimation error from a two-peg test for laser levels, builder's levels, and automatic levels. Shows instrument error in ft/100ft with pass/fail and adjustment instructions.
Two-Peg Test Setup
Set two pegs 100–200 ft apart on level ground. Take rod readings at each peg from both setups (instrument at each peg). The 4 readings reveal the instrument's collimation error.
Setup 1 — Instrument at Peg A
Setup 2 — Instrument at Peg B
Typical: 0.05 ft/100ft; precise work: 0.02 ft/100ft
Spectra Precision, Topcon RL-H5A, and Leica Rugby 680 laser levels with factory calibration certs at Express Tools.
Shop Express Tools →The two-peg test (also called a peg test or bubble tube test) is a field procedure to check the accuracy of a level instrument. Two pegs are set about 100–200 feet apart. Rod readings are taken at both pegs from each of two instrument setups. The four readings reveal any systematic collimation error — the angle between the line of sight and a truly horizontal plane.
Best practice: before every critical job, after any impact or rough transport, when readings seem inconsistent, and at the start of each season. Laser levels and automatic levels should be checked monthly if used daily. Topcon recommends monthly checks; Leica specifies quarterly at minimum. Consult your instrument manual for manufacturer-specific intervals.
Common field tolerances: general construction 0.05 ft/100ft (6mm/30m), road and site grading 0.02 ft/100ft (2mm/30m), precise leveling 0.005 ft/100ft (0.5mm/30m). Use the tighter tolerance for precision grade work and the general construction tolerance for most site grading and utility work. New instruments typically have tighter factory-spec tolerances.
Many automatic levels and rotating laser levels have field-adjustable screws for the compensator bubble and cross-hair position. After identifying the error with the two-peg test, set up at the midpoint between pegs, calculate the true reading, and adjust until the instrument reads correctly. Re-run the two-peg test to confirm. If the error cannot be eliminated in the field, the instrument needs service.
Yes. Extreme temperature changes — hot sun heating one side of the instrument body, or cold morning startup in warm storage conditions — cause temporary calibration shifts due to differential thermal expansion. Best practice: shade the instrument during setup, allow 15 minutes for thermal equilibration, and re-check calibration if temperature changes more than 20°F during the workday on precision projects.