Subgrade elevation is the designed level of the prepared ground that supports pavement, slab, or base layers. Getting it right helps control thickness, compaction effort, drainage performance, and long-term serviceability. On site, crews often work from a finished grade reference, then subtract the planned layer build-up to reach the formation. This calculator streamlines that workflow by converting mixed units, summarizing layer totals, and presenting a clear subgrade value you can share with supervisors and inspectors.
The most reliable starting point is a finished grade elevation taken from drawings, profiles, or set-out data. If the point grade is not directly listed, you can derive it from a centerline elevation and a cross slope (crossfall). With a known offset from centerline, the slope adjustment is computed as slope percentage multiplied by the offset distance. The direction selector applies a sign so the elevation increases or decreases based on which side the point lies. Because different projects may define “down to right” differently, always confirm the convention against your plans.
Layer thicknesses typically include pavement or slab depth, base course, and subbase, with optional capping or blanket layers used to improve weak soils. An allowance can be added when overbuild, undercut, trimming loss, or proof-rolling corrections are expected. The calculator totals these thicknesses and subtracts the converted value from the finished grade to obtain the subgrade elevation. When you also enter an existing ground level, it estimates cut or fill at subgrade, helping you anticipate earthworks and haul requirements.
Example: Suppose your finished grade at a driveway point is 102.350 m. You plan 200 mm pavement, 150 mm base, 200 mm subbase, and a 25 mm allowance. Total thickness is 575 mm (0.575 m). The computed subgrade elevation is 102.350 − 0.575 = 101.775 m. If existing ground is 101.900 m, the ground sits 0.125 m above subgrade, indicating approximately 0.125 m of cut to reach formation.
In practice, you can use the subgrade elevation to set stringlines, grade stakes, or machine control targets. Record spot checks along the alignment, especially at transitions, driveways, and drainage inlets. If thicknesses change by station, run separate calculations and keep them in your quality records to support pay items and as-built documentation.
Use the exported CSV for quick checking in spreadsheets and the PDF report for field packs. For best results, keep the same datum and coordinate system across surveys, drawings, and machine control files. Always verify critical levels with a licensed surveyor and project specifications before construction.