SPT Correlation Calculator

Convert a raw field SPT blow count into the standardized values used in design: energy-corrected N₆₀, overburden-normalized (N₁)₆₀, and correlated friction angle and relative density for granular soils.

Educational / preliminary estimating tool. Results use published correlations and idealized assumptions (homogeneous soil, no groundwater effects unless stated). They are not a substitute for a site-specific investigation and design by a licensed geotechnical engineer.

SPT Field Data & Corrections

Field Blow Count N

blows/ft

Hammer Energy E_m

Borehole Diameter C_B

Sampler C_S

Rod Length C_R

Effective Overburden σ′_v (optional)

psf

Enter a field N-value and hammer energy to compute corrected blow counts.

The corrections

N₆₀ = N · (E_m/60) · C_B · C_S · C_R · (N₁)₆₀ = C_N · N₆₀

Hammer energy is the dominant correction — safety hammers deliver roughly 60% of theoretical energy (the reference), automatic trip hammers often 80% or more. Borehole diameter, sampler liner, and rod length corrections follow Skempton (1986). The overburden factor uses Liao-Whitman, C_N = √(2000 psf / σ′_v), capped at 1.7, normalizing to one atmosphere.

The correlations

Friction angle uses Wolff (1989): φ′ ≈ 27.1 + 0.3·(N₁)₆₀ − 0.00054·(N₁)₆₀². Relative density follows Skempton’s (N₁)₆₀/D_r² ≈ 60 for normally consolidated sands. Both are sand correlations — SPT values in clays indicate consistency but do not convert to φ′ this way, and gravels can produce misleadingly high blow counts.

Frequently Asked Questions

What is the difference between N and N60?

Field N is the raw blow count; N60 standardizes it to 60% hammer energy efficiency with corrections for borehole size, sampler, and rod length, making values comparable across rigs.

What does (N1)60 mean?

It is N60 normalized to a standard overburden stress of one atmosphere (~2,000 psf / 100 kPa), removing the effect of depth so density correlations can be applied.

What N value indicates dense sand?

Using N60: below 4 very loose, 4–10 loose, 10–30 medium dense, 30–50 dense, over 50 very dense (Terzaghi & Peck descriptors).