Calculate horizontal seismic coefficient, peak ground acceleration, or gravitational acceleration from any 2 values with unit conversion.
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Horizontal Seismic Coefficient Formula
The horizontal seismic coefficient is the ratio of peak ground acceleration to gravitational acceleration. It is dimensionless because both values are accelerations.
Rearranged forms used by the calculator are:
- HSC = horizontal seismic coefficient, dimensionless
- PGA = peak ground acceleration
- g = gravitational acceleration
If you enter peak ground acceleration and gravitational acceleration, the calculator divides PGA by g to find the horizontal seismic coefficient.
If you enter the horizontal seismic coefficient and gravitational acceleration, it multiplies them to find peak ground acceleration.
If you enter the horizontal seismic coefficient and peak ground acceleration, it divides PGA by HSC to find gravitational acceleration.
The calculator converts acceleration units to meters per second squared before applying the formula, then converts the missing value back to the unit you selected.
Acceleration Unit Conversions
Use these values to check unit conversions when comparing results in m/s², ft/s², or g.
| Unit | Equivalent in m/s² | Common use |
|---|---|---|
| 1 m/s² | 1 m/s² | SI acceleration input |
| 1 ft/s² | 0.3048 m/s² | US customary acceleration input |
| 1 g | 9.80665 m/s² | Acceleration as a fraction or multiple of gravity |
Typical Horizontal Seismic Coefficient Values
These ranges are general reference values only. Project-specific seismic coefficients depend on the applicable code, site class, structure type, and design method.
| HSC value | Equivalent PGA | General interpretation |
|---|---|---|
| 0.05 | 0.05 g | Low horizontal acceleration |
| 0.10 | 0.10 g | Moderate horizontal acceleration |
| 0.20 | 0.20 g | High horizontal acceleration |
| 0.30+ | 0.30 g or higher | Very high horizontal acceleration |
Example Problems
Example 1: Find the horizontal seismic coefficient
Suppose the peak ground acceleration is 2.5 m/s² and gravitational acceleration is 9.80665 m/s².
The horizontal seismic coefficient is 0.2549.
Example 2: Find peak ground acceleration
Suppose the horizontal seismic coefficient is 0.20 and gravitational acceleration is 9.80665 m/s².
The peak ground acceleration is 1.9613 m/s², which is equal to 0.20 g.
FAQ
Is the horizontal seismic coefficient the same as PGA in g?
Yes, when gravitational acceleration is taken as 1 g, the horizontal seismic coefficient has the same numeric value as PGA expressed in g. For example, a PGA of 0.25 g gives an HSC of 0.25.
Why is the horizontal seismic coefficient dimensionless?
It is dimensionless because it is calculated by dividing one acceleration by another acceleration. The units cancel out, leaving a ratio rather than a value in m/s² or ft/s².
What value should I use for gravitational acceleration?
For most calculations, use standard gravity: 9.80665 m/s², 32.174 ft/s², or 1 g. If your problem or design standard gives a different value, use the value specified there.