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The energy-corrected field SPT blow count (already corrected to 60% hammer efficiency).
Most modern SPT equipment and reporting already applies the 60% energy correction — check your boring log.
The effective vertical stress at the test depth.
Computed from the soil unit weight(s) above the test depth, accounting for groundwater (buoyant unit weight below the water table).
Corrected N-value (N1,60)
21.2 (N1,60)
This applies overburden correction only — a complete SPT interpretation for liquefaction or bearing capacity analysis often needs additional corrections for borehole diameter, rod length, and sampler type, per the specific design method being used.
- Overburden correction factor (CN)
- 1.41
They open the calculator with your figures already in it
SPT N-Value Overburden Correction Calculator: 21.21 (N1,60) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Liao, S. S. C. and Whitman, R. V. (1986), Overburden Correction Factors for SPT in Sand, ASCE Journal of Geotechnical Engineering: CN = sqrt(Pa / σ'v), capped at 2.0; corrected N-value N1(60) = CN x N60, where Pa is atmospheric pressure (~100 kPa) and σ'v is the effective vertical overburden stress
Inputs used
- Field N60 Value
- 15
- Effective Overburden Stress (σ'v)
- 1044.27 psf
Intermediate steps
- Overburden correction factor (CN)
- 1.41
Confidence note: This applies overburden correction only — a complete SPT interpretation for liquefaction or bearing capacity analysis often needs additional corrections for borehole diameter, rod length, and sampler type, per the specific design method being used.
What this calculation does not cover
- This applies the overburden correction and nothing else. It assumes the value you type is already energy-corrected to 60 per cent hammer efficiency, and it does not apply the rod length, borehole diameter or sampler liner corrections a full interpretation needs. A log that quotes N without saying which correction it already carries leaves that question open — settle it before entering the number, because correcting a raw field N here puts two errors in one figure with no way to separate them.
- The overburden correction and N1,60 are granular-soil tools. A blow count taken in soft clay, silt or organic ground and corrected this way is arithmetic with no soil property behind it — cohesive strength comes from a field vane, an undrained triaxial or a piezocone sounding, not from a corrected N.
- N1,60 is an input to liquefaction and settlement procedures, not the assessment itself. It does not apply the fines-content adjustment to an equivalent clean-sand value, and it says nothing about cyclic stress ratio, earthquake magnitude, or the seismic demand at your site.
- The 2.0 ceiling on CN binds at any effective stress below 25 kPa, so every reading from the shallowest part of the log comes back as exactly twice the entered N60 and stops responding to depth at all. That ceiling is a practice convention rather than part of the original correction relation, and some procedures cap it lower — a shallow result here can sit above what those methods would allow.
- It takes the effective vertical stress as given. It does not derive it from depth, unit weights and the water table, and it does not check that you used buoyant unit weight below the water table — entering total stress instead of effective overstates the stress and understates the corrected N-value. The field is in kPa and does not follow the site's metric/imperial switch, so convert a stress quoted in psf or tsf before it goes in.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Liao, S. S. C. and Whitman, R. V. (1986), Overburden Correction Factors for SPT in Sand, ASCE Journal of Geotechnical Engineering: CN = sqrt(Pa / σ'v), capped at 2.0; corrected N-value N1(60) = CN x N60, where Pa is atmospheric pressure (~100 kPa) and σ'v is the effective vertical overburden stress
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