Basin Hydrostatic Test Calculator (ACI 350.1)

Basin Hydrostatic Test Calculator

Water-tightness (liquid loss) testing of concrete basins, tanks, and channels per the ACI 350.1 method: corrected water surface drop against the allowable percentage of volume per 24 hours. Runs entirely offline; inputs persist in this browser via localStorage.

Use limits: This computes the arithmetic of the test. The governing acceptance criterion (0.05% vs. 0.10% of volume per 24 hr, or something project-specific), the stabilization period, the test duration, and what happens on a failure all come from the contract specs and the referenced edition of ACI 350.1. Fill rates, temporary bulkheads, and disposal of test water are their own plan.

Basin Geometry at Test Water Level

Shape

Inside length (ft)

Inside width (ft)

Water depth at start of test (ft)

Test water level should be at the maximum operating level unless the spec says otherwise.

Test Measurements

Measured water surface drop (in)

From fixed reference gauges/points, not the same wet mark eyeballed twice.

Test duration (hr)

Evaporation / rainfall correction

Pan water surface change (in) — drop positive, rise negative

A calibrated open pan floating in the basin sees the same evaporation and rainfall as the basin. Net leakage drop = basin drop − pan change.

Acceptance Criterion

Allowable loss (% of volume per 24 hr)

The designation is on the drawings or in the specs — 0.05% is typical for environmental structures.

—

Water surface area / test volume	—
Corrected (leakage) drop	—
Volume lost to leakage	—
Loss rate	—
Allowable	—
Allowable drop over this test duration	—

How the Numbers Are Derived

The math

Corrected leakage drop:

Pan method: Δh = drop_basin − Δh_pan (the pan sees the same evaporation and rainfall, so subtracting it isolates leakage).
Split method: Δh = drop_basin − evaporation + rainfall.

Volume lost: V_lost = Δh/12 × A × 7.48 gal. Loss rate normalized to 24 hr:

%/24hr = V_lost / V_test × (24 / t_hr) × 100

The pre-test planning number — the allowable drop in inches over your test duration:

Δh_allow = (allow%/100) × V_test / (A × 7.48) × 12 × (t_hr/24)

Running the test so the numbers mean something

Stabilization first. New or long-dry concrete absorbs water; ACI 350.1 requires the basin sit full for a stabilization period (commonly 3 days, per the standard/spec) before the measured test starts, or an early "failure" is just the concrete drinking.
Measurement precision governs duration. If the allowable drop over 24 hr is smaller than you can reliably measure (roughly 1/16 in. with a good hook gauge or fixed staff), extend the test — the standard allows longer durations for exactly this reason. The tool flags when the allowable drop is under 0.25 in.
Fixed reference points. Measure from set gauges or marked points at multiple locations, at the same time of day (thermal expansion of the water is real over a big surface).
Valves and bulkheads count as leaks. A weeping plug valve or temporary bulkhead fails the basin on paper. Isolate and verify them before blaming the concrete.
Damp spots vs. flowing water. Most specs distinguish acceptable dampness from unacceptable flowing/trickling leakage on the exterior regardless of the volumetric result — walk the walls, don't just read the gauge.

References

ACI 350.1 — Specification for Tightness Testing of Environmental Engineering Concrete Containment Structures. https://www.concrete.org/

AWWA D110 / D115 — tightness testing provisions for circular prestressed tanks, where applicable.

Contract specifications — governing criterion, stabilization period, and retest provisions.