Size a precast manhole in seconds. Pick your pipes and bearings; the tool solves the minimum inner diameter and the next standard precast size (48, 60, 72, 84, 96 in). No signup. Works on mobile.
Pipes (2)
Pipe 1
Pipe 2
Recommended precast MH
48in ID
Fits
Minimum required inner diameter: 32.8 in · margin above required: 15.3 in
Adjacent pair checks
2 → 1
90.0° between axes · arc gap 6.1 in · required 6.0 in
+0.1 in tight
1 → 2
270.0° between axes · arc gap 57.5 in · required 6.0 in
+51.5 in
How the math works
The tool sizes the minimum MH inner diameter needed so that no two adjacent pipe holes clash on the MH wall. For each pair of adjacent pipes (sorted by bearing), it computes the angular width each pipe hole occupies as delta = 2 arcsin(OD / (2R)), subtracts each pipe half-width from the axis-to-axis angular separation, converts the remaining angular gap to an arc length R x gap_rad, and requires that arc length to be at least the min clearance. The solver increments R by 0.125 in until every pair passes.
OD lookup by material: RCP uses ACPA Wall B (ASTM C76). PVC uses SDR 35 (ASTM D3034). HDPE uses AASHTO M294 dual-wall. DIP uses AWWA C151 Class 52. You can also override OD manually per pipe.
Standard precast IDs matched from NPCA reference: 48, 60, 72, 84, 96, 108, 120, 144, 168 in. Recommended size is the smallest standard at or above the computed minimum.
Caveat. First-pass sizing check. Structural design (wall thickness, base slab, load rating, cone eccentricity) is your engineer's call. Regional DOT standards may require larger MH than the geometric minimum for maintenance access; cross-check your local DOT sizing chart.
Built by ARC
ARC does 40 of these on a full civil plan PDF in 8 minutes.
Deflect calcs, takeoffs, submittal drafts, all from a flat PDF the civil engineer just sent you. Already in daily production at one of the top 5 US precast shops.