Calculate fresh concrete fluid-head pressure, equivalent head from a pressure rating, and a qualified placement-rate estimate. This calculator does not approve formwork or establish a safe pour rate.
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Concrete Pressure Formula
The calculator uses the hydrostatic relation and its inverse, with a limited ACI 347R-14 model for eligible conventional placements.
Pressure from height (full hydrostatic):
Height from rating (inverse hydrostatic):
Placement-rate check (limited ACI 347R-14 model):
- 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 = verified concrete chemistry coefficient (1.0, 1.2, 1.4 or 1.5)
- Cw = concrete unit-weight coefficient from ACI 347R-14
The reduced-pressure model requires conventional top placement, slump ≤ 7 in after admixtures and internal vibration depth ≤ 4 ft. Equation b applies to qualifying columns (every plan dimension ≤ 6.5 ft) and walls ≤ 14 ft at less than 7 ft/hr. Equation c applies to taller walls below 7 ft/hr or walls at 7–15 ft/hr. Faster wall placement, SCC or unconfirmed chemistry uses hydrostatic pressure. Cc must be verified from the mixture; flow-class multipliers do not establish it. Cw reflects unit weight. Base pumping requires additional surge allowance, and external vibration or expansive cement can exceed hydrostatic pressure.
The Pressure from height mode reports the hydrostatic reference and triangular resultant. Equivalent head from rating gives the fluid head corresponding to a rating; it is not an approved maximum pour height. Placement-rate pressure check applies a reduced-pressure model only under its stated conditions, otherwise it reports a hydrostatic reference or a specific limitation.
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. For an eligible top-placed conventional 8 ft wall at 5 ft/hr and 70 °F, use γ = 150 lb/ft³, verified Cc = Cw = 1, slump no more than 7 in after admixtures and internal vibration depth no more than 4 ft.
- Hydrostatic cap: 150 × 8 = 1200 lb/ft²
- Rate estimate: 1.00 × (150 + 9000 × 5 / 70) = 1.00 × (150 + 642.9) = 793 lb/ft²
- Limited-model pressure: max(600, 793), capped by 1200, gives about 793 lb/ft². Project-specific formwork checks remain required.
Why does pressure stop increasing with height in tall walls? Setting can reduce pressure in eligible conventional placements. This calculator uses that effect only within the stated ACI model conditions; uncertain setting behavior requires the hydrostatic reference or a project-specific assessment.
When should you use the full hydrostatic value instead? SCC and unconfirmed conditions use full hydrostatic pressure here. Base pumping includes a minimum 25% surge allowance; external vibration or expansive cement may exceed hydrostatic pressure and are not estimated by this tool.
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? Temperature affects setting in the limited rate model. This calculator applies reduced pressure only above 35 °F and uses hydrostatic pressure at lower temperatures; that cutoff is a calculator scope restriction, not a stated ACI temperature limit.
Does this replace a formwork design? No. Do not use this result alone to size panels, ties or shores or approve a pour. The formwork designer must account for all reasonably anticipated vertical and lateral loads, placement effects and the applicable project requirements.
