SPT N60 Correction Calculator

About this calculator

The Standard Penetration Test (SPT) is a geotechnical field test where a standard sampler is driven into the soil by a hammer, and the number of blows needed to advance it (N) is used to estimate soil density and strength. Because hammer energy delivery, borehole diameter, sampler type, and rod length all vary between rigs and drillers, raw N values aren't directly comparable — this calculator corrects the measured N to a standardized N60 value (the count at 60% of theoretical free-fall hammer energy) using N60 = N × (ER/60) × Cb × Cs × Cr.

N60 corrections are standard practice for geotechnical engineers and foundation designers, who use the corrected blow count — rather than the raw field number — to estimate bearing capacity, settlement, and liquefaction potential when designing footings, piles, and earthworks. Comparing N60 values across different boreholes and drilling rigs is only meaningful once each has been normalized this way.