Infiltration Trench Sizing Calculator

Where infiltration trenches earn their keep

Dig a narrow slot, fill it with clean single-size stone, and wrap it in filter fabric: that is an infiltration trench in a sentence. The gaps between the stones do the work. During a storm they fill with runoff, and between storms the water seeps out through the trench floor into the native soil below. Because roughly two-thirds of a stone-filled trench is solid rock, the storage is compact but real, and it all sits below grade — which is exactly why designers reach for trenches along parking-lot edges, beside driveways, under a strip of lawn, or against a building where a surface pond would never fit.

The three numbers this page returns answer the three questions you actually face at the sketch stage. How much water are we trying to catch for the storm we picked? How long a trench does it take to hold that water in the stone voids? And once it is full, will it empty out before the next storm arrives? Everything here is planning-grade arithmetic you can check by hand — deliberately simple, so you can see which input is driving the design instead of trusting a black box.

What each field is asking for

1. Enter the drainage area that contributes runoff to the trench (for example, a roof area or paved area). Use square meters (m²).
2. Enter the design rainfall depth in millimeters (mm). This is the storm depth you want the trench to capture.
3. Enter trench width and depth in meters (m). These are the internal dimensions of the aggregate storage zone.
4. Enter the aggregate void ratio (between 0 and 1). This represents the fraction of the trench volume that is empty space available for water storage. Typical clean stone is often around 0.35–0.50.
5. Enter the soil infiltration rate in mm/hr. Use a measured value if available (field tests are best). If you only have a rough soil texture, use the reference table below as a starting point.
6. Click Calculate to see runoff volume, required trench length, and estimated drain time.

Tip: If the drain time is very long, consider increasing bottom area (wider trench, multiple trenches, or a shallower design) or reassessing the infiltration rate with site testing. Many local standards target drain-down within about 24–48 hours to reduce prolonged saturation.

How the three numbers are calculated

Internally everything runs in metres and metres-per-hour. Rainfall depth and infiltration rate come in as millimetres and get divided by 1000 before they touch the math, so you never have to juggle mixed units. The three equations are deliberately plain — no runoff routing, no unsaturated-flow model — so you can reproduce every result on the back of an envelope.

Step 1 — Runoff volume

Multiply the contributing area by the storm depth. That is the whole of it:

V is runoff volume (m³), A_d is the drainage area (m²), and D_r is rainfall depth in metres. Treating every millimetre of rain as runoff is the same as assuming a runoff coefficient of 1.0 — reasonable for a roof or asphalt, generous for anything with grass or gravel in it. Where part of the catchment is pervious, shave the area down to an effective value before you enter it (see the FAQ).

Step 2 — Storage per metre and required length

A one-metre slice of trench holds only the water that fits in the stone voids: its cross-section times the void ratio.

B is width (m), Z is depth (m), and n is the void ratio (a bare fraction). Divide the storm volume by that per-metre storage and you have the length the trench needs to be:

Step 3 — Drain-down time

When the trench is full, the water sitting in the voids adds up to a column of depth Z×n of actual water. That column drains through the trench floor at the soil's infiltration rate, so the emptying time is simply the water depth divided by that rate:

t_d is time in hours, f is the infiltration rate (m/hr), Z is depth, and n is the void ratio. The width cancels out, which surprises people: because both the stored water and the infiltrating floor scale with width, a wider trench drains in the same time — it just holds more. What actually stretches the drain time is a deep trench over slow soil, and that is the combination worth watching.

Sizing a trench for a 200 m² roof

Say a 200 m² roof feeds the trench and you are designing for a 25 mm storm. Step 1 gives the volume you have to swallow:
V = 200 × 0.025 = 5.0 m³.

Build the trench 0.6 m wide, 1.0 m deep, and fill it with clean gravel (n = 0.40). Each metre then stores
S_1m = 0.6 × 1.0 × 0.40 = 0.24 m³,
so the run has to be
L = 5.0 / 0.24 ≈ 20.8 m.

Now the reality check. On loam draining at 10 mm/hr (0.01 m/hr), the full trench empties in
t_d = (1.0 × 0.40) / 0.01 = 40 hours.
That is right at the top of the 24–48 hour window most manuals aim for — workable, but with no margin for the infiltration rate being optimistic. Drop the depth to 0.6 m and it empties in about 24 hours (at the cost of roughly 1.6× more length), which is the trade this calculator is built to let you feel.

Reference values (starting points)

Use measured site data whenever possible. The values below are only rough planning-level ranges and can vary widely with compaction, layering, groundwater conditions, and clogging potential.

Interpretation guide (what the results mean)

The calculator returns three values: runoff volume, required trench length, and estimated drain time. Each value answers a different design question. Runoff volume is the amount of water you are trying to manage for the selected storm depth. If you increase the rainfall depth or the drainage area, the volume increases linearly. Required trench length is the length of trench needed to provide enough void storage to hold that volume at once. Length decreases if you increase width, depth, or void ratio, because each meter of trench stores more water. Estimated drain time is a simplified indicator of how quickly the stored water can infiltrate into the soil.

A common planning check is whether the drain time is within a target window such as 24 to 48 hours. Faster drain-down reduces the chance of prolonged saturation and helps the trench recover storage capacity before the next storm. However, extremely fast infiltration is not always better: in sensitive groundwater areas, designers may need additional treatment or separation distances. Treat the drain time as a screening metric and confirm with local requirements.

Practical sizing tips

If the required length is impractically long, you can often adjust the design without changing the captured storm depth. Increasing width is usually the most straightforward way to add storage per meter, but it requires more footprint. Increasing depth adds storage too, but deeper excavations can encounter groundwater, utilities, or unsuitable soils. Increasing void ratio is limited by the aggregate type; clean, uniformly graded stone tends to have higher void space than well-graded mixes.

If the drain time is too long, the controlling factor is typically the infiltration rate. In low-permeability soils, a trench may need to be shallower, wider, or split into multiple parallel trenches to increase the infiltrating area. In some projects, an underdrain or a lined system with controlled discharge is used instead of full infiltration. Also consider pretreatment: sediment is the most common cause of long-term performance loss, so a filter strip, sump catch basin, or forebay can protect the trench.

Common unit checks and input guidance

Unit mistakes are a frequent source of unrealistic results. Rainfall depth is entered in millimeters, so a 1-inch storm is about 25.4 mm. Infiltration rate is also entered in mm/hr. If you have an infiltration rate in inches per hour, multiply by 25.4 to convert to mm/hr. Trench width and depth are entered in meters; for example, 24 inches is about 0.61 m.

The void ratio input should be a decimal fraction between 0 and 1 (for example, 0.40), not a percentage (40). If you enter 40, the calculator will reject it because it is outside the valid range. If you are unsure, start with 0.40 for clean gravel and refine later based on material specifications.

Limitations and design notes

This tool is intentionally simplified. Use it for preliminary sizing and education, not as a final engineering design. Key limitations include:

• Runoff coefficient not included: The runoff volume calculation assumes the full rainfall depth becomes runoff from the drainage area. Real designs often apply a runoff coefficient (C) or use hydrologic modeling.
• Constant infiltration rate: Soil infiltration can decrease as the soil becomes saturated, and it can vary by depth due to layering. Field testing (e.g., double-ring infiltrometer) is recommended.
• Clogging and maintenance: Sediment and fines can reduce performance over time. Pretreatment (filter strip, swale, catch basin) and maintenance are critical. Consider a safety factor or additional length to account for long-term degradation.
• Groundwater and separation: Many jurisdictions require vertical separation to seasonal high groundwater or bedrock. Always check local codes and site constraints.
• Water quality considerations: Infiltration may be inappropriate where runoff contains high pollutant loads or where groundwater protection is required.
• Geometry simplification: The calculator treats the trench as a uniform rectangular prism and does not account for sidewall infiltration, pipe distribution, overflow structures, or underdrains.

If your calculated drain-down time is much greater than 48 hours, consider alternative stormwater practices (rain garden/bioretention with engineered soil, detention with controlled release, or a lined system) or consult a qualified professional.

Questions that come up during design

Does this calculator include a runoff coefficient?

No. The runoff volume is computed as drainage area multiplied by rainfall depth, which is equivalent to assuming a runoff coefficient of 1.0. If you want a quick adjustment for partially pervious drainage areas, you can approximate by reducing the drainage area to an “effective” area (for example, multiply the actual area by your chosen coefficient) before entering it.

Why is the drain time so sensitive to infiltration rate?

In the simplified drain-down equation, infiltration rate appears in the denominator. Cutting the infiltration rate in half doubles the estimated drain time. Because infiltration rates can vary by orders of magnitude between sands and clays, it is normal to see very different drain times for the same trench geometry.

What if my project requires overflow?

Many trenches are designed with an overflow to safely pass storms larger than the design event. This calculator sizes storage for the selected storm depth only; it does not design overflow structures. In practice, you would provide a stabilized overflow path or connection to a downstream conveyance system.