Calculate fresh concrete formwork pressure from height, find equivalent head from pressure rating, and check pour rate limits safely.
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Concrete Pressure Formula
The calculator uses three formulas, one for each tab.
Pressure from height (full hydrostatic):
P = γ × h
Height from rating (inverse hydrostatic):
h = P / γ
Pour rate check (ACI 347 style screening):
P = Cc × (150 + 9000 × R / T), capped by γ × h, minimum 600 lb/ft²
- P = lateral pressure on the formwork (lb/ft²)
- γ = unit weight of fresh concrete (lb/ft³)
- h = depth of fresh concrete above the point of interest (ft)
- R = vertical placement rate (ft/hr)
- T = concrete temperature at placement (°F)
- Cc = consistency or flow class factor (0.85 to 1.25)
Assumptions: fresh concrete behaves as a fluid until it sets, vibration is internal and limited to the top placement layer, and pour height is measured from the top of the fluid concrete to the point being checked. The rate-based formula is a screening approximation aligned with ACI 347 column and wall provisions; it is not a substitute for a stamped formwork design. The minimum of 600 lb/ft² reflects the practical floor used in ACI 347 for walls and columns. Self-consolidating concrete is treated as full hydrostatic in most codes; the SCC factor here is a conservative rate-check only.
The Pressure from height tab applies P = γh directly and reports the pressure at the bottom plus the triangular resultant per foot of wall. The Height from rating tab solves the same equation for h so you can see the maximum fluid head a rated panel can carry. The Pour rate check tab runs the ACI rate formula, compares it to the hydrostatic cap, and returns whichever value is lower as the design pressure.
Reference Tables
Typical fresh concrete unit weights used in formwork calculations:
| Concrete type | Unit weight (lb/ft³) | Unit weight (kN/m³) |
|---|---|---|
| Lightweight | 110 to 120 | 17.3 to 18.8 |
| Typical fresh normal weight | 145 | 22.8 |
| Normal weight (design) | 150 | 23.6 |
| Heavyweight (barite, magnetite) | 200 to 240 | 31.4 to 37.7 |
Full hydrostatic pressure at common pour heights, using γ = 150 lb/ft³:
| Height (ft) | Pressure (lb/ft²) | Pressure (psi) | Pressure (kPa) |
|---|---|---|---|
| 2 | 300 | 2.1 | 14.4 |
| 4 | 600 | 4.2 | 28.7 |
| 6 | 900 | 6.3 | 43.1 |
| 8 | 1200 | 8.3 | 57.5 |
| 10 | 1500 | 10.4 | 71.8 |
| 15 | 2250 | 15.6 | 107.7 |
| 20 | 3000 | 20.8 | 143.6 |
Worked Example and FAQ
Example. You are pouring an 8 ft wall at 5 ft/hr with concrete at 70 °F, F3 consistency, and γ = 150 lb/ft³.
- Hydrostatic cap: 150 × 8 = 1200 lb/ft²
- Rate estimate: 1.00 × (150 + 9000 × 5 / 70) = 1.00 × (150 + 642.9) = 793 lb/ft²
- Design pressure: the lower value, 793 lb/ft², governs.
Why does pressure stop increasing with height in tall walls? Once concrete begins to set, the lower layers stiffen and stop transmitting full fluid pressure. The rate-based formula approximates that effect. Slower pours and warmer concrete reduce the design pressure.
When should you use the full hydrostatic value instead? Use full hydrostatic pressure for self-consolidating concrete, very rapid pours, retempered mixes, externally vibrated forms, and any pour where set times are uncertain. Most codes require it for SCC.
What rate counts as the placement rate? It is the vertical rise of fresh concrete in the form per hour, not the truck discharge rate. If a 10 ft wall is filled in 2 hours, R = 5 ft/hr.
Why does temperature appear in the formula? Cooler concrete sets more slowly, so a larger column of it stays fluid at once. Lower T gives higher pressure. Do not use temperatures at or below freezing in this calculator.
Does this replace a formwork design? No. Use the result as a screening number to size panels, ties, and shores or to compare against a manufacturer's rated pressure. Final formwork design should be signed by a qualified engineer.
