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.

What is the Garden Soil Drainage Percolation Calculator?

Enter the water level before and after a timed interval in your test hole to compute the percolation rate in inches or centimetres per hour and get a drainage class — fast, moderate, slow, or poor — for siting beds and plants. It runs free in your browser on Gera Tools, with nothing uploaded.

Garden Soil Drainage Percolation Calculator

Name: Garden Soil Drainage Percolation Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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

Step-by-step method

Dig the hole. Aim for roughly 12 inches (30 cm) deep and a similar diameter. A post-hole digger makes clean work of this.
Pre-soak. Fill the hole with water and let it drain completely. This saturates the surrounding soil so the test reflects steady-state drainage — not the unusually fast absorption of dry ground. If the soil drains in under 10 minutes it is probably fast-draining; if it takes more than an hour to empty, you may already be looking at poor drainage.
Refill and time. Fill the hole again to a measured start depth (from the surface or from a marker stick) and start a timer. A 15- or 30-minute interval works for most soils; use 60 minutes for very slow draining sites.
Measure the drop. Note the water depth at the end of your interval and subtract it from the start depth to get the drop. Enter both values and the elapsed time into the calculator.

What to do with each drainage class

Fast draining (above about 6 in/hr): Sandy soils lose water quickly. Roots of most vegetables and ornamentals need consistent moisture, so fast drainage calls for adding organic matter — compost or well-rotted manure dug in generously — to improve water-holding capacity. Drought-tolerant Mediterranean plants will thrive here without intervention.

Moderate (1–6 in/hr): This range suits the widest range of plants and is the target for most vegetable gardens, lawns, and mixed beds. No structural intervention needed; focus on regular soil health maintenance.

Slow (0.25–1 in/hr): Clay-heavy soils that drain slowly will puddle after heavy rain and can leave roots waterlogged for extended periods. Raised beds are the most reliable fix. In-ground, working in coarse horticultural grit (not builders’ sand) and compost improves structure over time. Avoid compacting slow-draining soil by working it only when slightly moist.

Poor (below 0.25 in/hr): This often indicates a clay hardpan layer — a compacted layer below the surface that impedes drainage regardless of what the topsoil looks like. Check by testing at root depth, not just at the surface. Hardpan can sometimes be broken up with a long-tined fork or subsoiler, but raised beds above an impermeable layer are frequently the more practical solution. Bog-loving plants and marginal pond plants are the only in-ground option that actively benefits from this condition.

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 — free-draining topsoil can sit over an impermeable clay layer that the surface test completely misses. Run two or three tests in different spots across the bed to catch spatial variation, especially in gardens where previous use involved compaction.