How do I run a percolation test?

Dig a hole about 30 cm deep, fill it with water and let it drain fully once to pre-soak the soil. Refill it, then record the starting water depth, wait a fixed time such as 15 minutes, and measure how far the level dropped.

Why pre-soak the soil first?

Dry soil absorbs the first fill abnormally fast, giving a misleadingly good drainage reading. Pre-soaking saturates the surrounding ground so the measured test reflects steady-state percolation, which is what your plant roots actually experience.

What drainage rate is good for most plants?

A moderate rate of roughly 1 to 6 inches (2.5 to 15 cm) per hour suits most garden plants and raised beds. Faster than that dries out too quickly and needs more watering or organic matter; slower than about half an inch per hour risks root rot.

What does a poor result mean?

A rate under about 0.25 inch (around half a centimetre) per hour signals heavy clay or a hardpan that holds water and starves roots of oxygen. Amend with grit and compost, build raised beds, or choose bog-tolerant plants.

Does hole size affect the result?

The rate is measured as the fall of the water level over time, so it is independent of hole diameter. It does depend on depth and soil layering, so test at the actual root depth of what you plan to grow.

Garden Soil Drainage Percolation Calculator

A simple percolation test tells you how fast water moves through your soil, which decides whether a spot will drown roots or dry out too fast. This calculator turns a before-and-after water-level reading into a drainage rate and a plain-English class.

How it works

After pre-soaking the hole, you measure how far the water level falls over a set time:

drop          = start depth − end depth
rate per hour = drop ÷ (minutes ÷ 60)

The rate is then matched to a drainage class:

fast      ≳ 6 in/hr   (15 cm/hr)   — sandy, dries quickly
moderate  1–6 in/hr   (2.5–15 cm)  — ideal for most plants
slow      0.25–1 in/hr              — heavy loam/clay, water carefully
poor      < 0.25 in/hr              — clay/hardpan, amend or raise beds

Notes and tips

Always do the one-time soak first, or sandy-looking soil will read far better than it drains in practice. Test at the depth your plants will root, since a free-draining topsoil can sit over an impermeable clay layer. If you land in the slow or poor band, working in coarse grit and compost, or switching to raised beds, lifts the effective drainage; in the fast band, adding organic matter improves water retention.