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SPT N60 Correction Calculator

Enter the measured N value and correction factors.

How it is calculated

The corrected blow count is N60 = N × (ER / 60) × Cb × Cs × Cr. N is the field blow count over the test interval, ER is the hammer energy ratio in percent (the tool divides it by 60, the reference energy), and Cb, Cs and Cr are dimensionless factors for borehole diameter, sampler type and rod length. The tool shows three figures: N60 rounded to one decimal, the energy factor ER / 60 to two decimals, and the combined factor Cb × Cs × Cr to four decimals. It does not choose Cb, Cs or Cr for you; take them from the correction table your design code or local practice uses. N must be zero or more, ER must be above 0 and at most 100, and the three correction factors must each be above zero; otherwise there is no result.

Worked example

A field test gives N = 18 blows with a hammer that delivers 72% energy, Cb = 1.05, Cs = 1.00 and Cr = 0.85. Energy factor: 72 / 60 = 1.20. Combined correction: 1.05 × 1.00 × 0.85 = 0.8925. Corrected count: 18 × 1.20 × 0.8925 = 19.3 blows. The higher-energy hammer pushes the value up, while the short-rod factor pulls it back down.

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Common mistakes

  • Entering the energy ratio as a fraction (0.72) instead of a percentage (72). The tool accepts it and divides by 60, so N60 comes out about one hundred times too small (0.2 instead of 19.3 here).
  • Applying the correction twice by feeding in a value that is already N60 or already energy-corrected.
  • Mixing up the factors: Cb is for the borehole, Cs for the sampler and Cr for the rod length. Swapping a rod-length value into Cb gives the same product but hides the error when the table changes.
  • Using the rod-length factor for the wrong depth. Cr depends on the rod length at the test depth, so the same boring needs a different Cr at each sampling level.
  • Entering an energy ratio above 100% or a correction factor of zero or less. These give no result; energy ratios are percentages of the theoretical free-fall energy.

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.

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