NBC 105:2025 Equivalent Static Seismic Coefficient Calculator

Equivalent static seismic coefficient calculator with live updates, cleaner grouped inputs, and sample values loaded by default.

Auto calculation Sample data loaded ESM only

Inputs

Update any value and the seismic coefficients refresh instantly.

Units: H in m, T in s

Analysis setup

Default spectrum values come from the selected soil type for equivalent static analysis.
Enter the PGA value for the project location.
Use the appropriate factor from the code importance class.
This tool is restricted to Equivalent Static Method (ESM) only.

Building and system

Measured from base to roof level.
Typical RC moment frame value: 0.075.
System factors
Keep auto-fill on to update Rμ, Ωu, and Ωs from the selected structural system.

Response spectrum parameters

Table 4-1 defaults
Use automatic values from the selected soil type for ESM, or switch off auto-fill to edit manually.
For ESM, Ta is taken as 0 as noted in NBC 105:2025 Table 4-1.
Wall load and Rayleigh period have been separated into their own calculators for a cleaner workflow.

Optional drift and displacement checks

These checks stay live as you update the seismic parameters or ETABS output values.

Results

Core coefficients update automatically from the current input values.

Live output
Approximate fundamental period, T = 1.25 × kt × H3/4
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Spectral shape factor, Ch(T)
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Elastic site spectrum, C(T) = Ch × Z × I
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Vertical spectrum factor, Cv = 2/3 × Z
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SLS spectrum, Cs(T) = 0.2 × C(T)
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Base shear coefficient (ULS), Cd = C(T) / (Rμ × Ωu)
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Base shear coefficient (SLS), Cd = Cs(T) / Ωs
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Drift and displacement checks

Quick pass/fail review using your ETABS output values.

Optional check
Allowable drift ratio (ULS)
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Allowable drift ratio (SLS)
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Allowable top displacement (ULS), mm
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Allowable top displacement (SLS), mm
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Calculation log 
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Always verify the final design inputs, spectrum values, and code assumptions with NBC 105:2025 and your project-specific structural model.