Inputs
Formula used
Required plan area (bearing-based):
A_req = (T * FS) / q_allow
Rectangular sizing with chosen ratio r = L/W:
W = sqrt(A / r) and L = r * W
Resisting weights:
W_conc = (A * t) * gamma_conc and W_soil = (A * cover) * gamma_soil
Uplift safety factor:
FS_uplift = (W_conc + W_soil) / T
Sliding resistance (if H is provided):
R = mu * (W_conc + W_soil) and FS_slide = R / H
Notes: 1 kPa equals 1 kN/m^2, so area comes out in m^2 in metric. This tool is a sizing aid; confirm with project design standards.
How to use this calculator
- Select your unit system and keep all inputs consistent.
- Enter tensile load T and your allowable soil bearing pressure.
- Set a bearing safety factor and choose an L/W ratio.
- Provide thickness and soil cover to evaluate resisting weight.
- Optionally add horizontal load H to check sliding safety.
- Click Calculate to see results above the form instantly.
- Download CSV or PDF to share with your team.
Example data table
| Scenario | T | q_allow | FS | t | Cover | L/W | Typical output |
|---|---|---|---|---|---|---|---|
| Compact footing area | 250 kN | 150 kPa | 1.50 | 0.60 m | 0.50 m | 1.50 | Area about 2.50 m^2, uplift FS depends on weights |
| Lower soil capacity | 250 kN | 90 kPa | 1.50 | 0.60 m | 0.50 m | 1.50 | Larger area required to reduce bearing |
| Sliding check included | 250 kN | 150 kPa | 1.50 | 0.60 m | 0.50 m | 1.50 | Enter H to compute sliding safety factor |
Professional guide: sizing anchor blocks in construction
Anchor blocks are used to resist uplift and movement at equipment bases, tiebacks, guyed structures, pipe thrust points, and temporary works. A practical sizing workflow begins with the governing load case, then checks bearing capacity, uplift, and sliding so the block performs reliably under site conditions. This calculator is built as a fast estimator that helps you iterate dimensions and see the effect of soil capacity, block thickness, cover depth, and friction on safety margins.
Start by defining the design tensile load T that the block must resist. For preliminary sizing, bearing-based plan area provides a clear first step: increasing allowable bearing pressure reduces area, while increasing safety factor increases area. After the plan area is estimated, the calculator selects a rectangular footprint using your chosen L/W ratio, enforces a minimum side constraint, and rounds dimensions up to a practical increment for formwork and excavation.
Next, the block’s resisting weight is evaluated. Concrete self-weight is computed from plan area and thickness, and soil cover weight is added when cover depth is provided. Comparing total resisting weight to the tensile demand yields an uplift safety factor, which helps you judge whether additional thickness, footprint, or cover is needed. If a horizontal load H is entered, the calculator also checks sliding using the friction resistance μW. These checks support better early decisions before detailed design drawings and reinforcement layout are finalized.
Example (metric): assume T = 250 kN, qallow = 150 kPa, FS = 1.50, t = 0.60 m, cover = 0.50 m, μ = 0.50, and L/W = 1.50. The required area is approximately A = (250 × 1.50) / 150 = 2.50 m². One feasible rounded solution is L = 1.95 m and W = 1.25 m (area 2.4375 m²) or slightly larger depending on your rounding increment and minimum side. The resulting uplift factor depends on the combined concrete and cover weights computed from unit weights.
In practice, also consider excavation stability, groundwater, and drainage so the block is not undermined or buoyant. Maintain adequate embedment and protect against frost heave where applicable. Keep reinforcement cover and lifting points in mind for constructability, and verify that any connected hardware transfers load into the block without crushing or excessive local stresses. Where loads are cyclic or dynamic, consult project criteria for additional factors.
Use this tool for preliminary sizing and comparison studies, then confirm the final design with your project geotechnical report, detailing standards, and applicable safety factors and load combinations.
FAQs
It sizes the plan area from allowable bearing using the tensile load and a bearing safety factor, then proposes length and width using your selected L/W ratio and rounding increment.
Soil over the block adds dead weight that helps resist uplift. Including cover depth can significantly increase resisting weight, especially for larger plan areas.
Pick a ratio that suits excavation constraints, equipment layout, and constructability. Common preliminary ratios range from 1.2 to 2.0, but site geometry often governs.
Use values from project specifications or testing when available. In preliminary work, typical ranges are about 0.3–0.6 depending on interface roughness and moisture conditions.
No. It is an estimator for footprint and basic safety checks. Final design should include reinforcement detailing, load combinations, uplift and overturning checks, and code compliance.
Increase plan area by adjusting the minimum sides, ratio, or rounding increment, or revisit the allowable bearing value with geotechnical input. The bearing utilization output helps you track this.
Yes. Select Imperial and enter loads in kip, pressure in psf, dimensions in ft, and unit weights in pcf. The calculator converts internally and reports results in the chosen units.
Build safer anchors by sizing blocks with confidence today.