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SPT N-Value Overburden Correction Calculator

Correct a field Standard Penetration Test (SPT) N-value for overburden pressure, per the Liao-Whitney method.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Corrected N-value (N1,60)

21.21 (N1,60)

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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

For the dimensions entered, expect a corrected n-value (n1,60) of 21.2. Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Liao and Whitney (1986) overburden correction: 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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Frequently asked questions

Why does the N-value need overburden correction?
The same soil density gives a higher blow count at greater depth simply because of the extra confining pressure — normalizing to a standard reference overburden (1 atmosphere) lets you compare N-values from different depths on equal footing.
Why is CN capped at 2.0?
The correction factor formula would grow indefinitely for very shallow (low overburden) tests, producing physically unrealistic results — the widely-used 2.0 cap keeps the correction from over-amplifying near-surface readings.
What is N1,60 used for?
It's the standard normalized blow count used in most simplified liquefaction assessment procedures and several bearing capacity and settlement correlations for granular soils.