Permeable Paver Calculator

How the permeable paver calculator works

Enter the paved area, the laying size of one block, the total depth of stone under the pavers, a wastage allowance and the price of one block. The calculator divides the area by the block's laying area, adds the wastage and rounds up to whole blocks. It then splits the stone depth into a bedding layer, a base layer and a sub-base, gives the volume of each, and estimates the weight of stone to order.

The block formula is N = ⌈A ÷ (L × W) × (1 + w)⌉. On the US units side the block is in inches, so its area is divided by 144 to get square feet; on the metric side it is in millimetres and divided by 1,000,000 to get square metres. Each stone volume is the area times that layer's depth.

Why the laying size matters

The US EPA describes permeable interlocking pavers as blocks that let rain and snowmelt seep between them into the layers of soil and gravel below. With interlocking pavers the water gets through the gaps between the blocks. CMHA Tech Spec 18, the US industry guide to building these pavements, keeps the spacing between units to ½ in (12 mm) or less and fills the joints with small crushed stone, ASTM No. 8, 89 or 9. Because each block also covers its share of joint, enter the block size plus one joint, or check the result against the units per square foot or metre in the manufacturer's data.

The difference is large. A 430 ft² drive in 8 × 4 in blocks with 7% wastage needs 2,071 blocks if the joints are ignored, since each block covers 32 in², or 0.222 ft². With ⅜ in joints the laying size is 8⅜ × 4⅜ in, about 36.6 in² or 0.254 ft², and the count drops to 1,809. In metric, 40 m² of 200 × 100 mm blocks with 7% wastage is 2,140 blocks without joints and 1,853 with 10 mm joints, which make the laying size 210 × 110 mm.

The stone layers

A permeable pavement stands on open-graded crushed stone: clean stone in a limited range of sizes, with no fines, so there are gaps between the pieces for water. CMHA Tech Spec 18 describes a 2 in (50 mm) bedding layer of ASTM No. 8 stone, a 4 in (100 mm) base of No. 57 stone laid in one lift, and a sub-base of larger No. 2 stone laid in lifts of up to 6 in (150 mm). The Minnesota Stormwater Manual gives the same bedding and base and recommends at least 40% voids in the stone reservoir. The calculator uses 2 in and 4 in on the US side, 50 mm and 100 mm on the metric side, and counts the rest of the depth you enter as sub-base.

In England, the government guidance on permeable front gardens warns against the usual driveway sub-base, MOT Type 1, because its fine material stops water passing through, and names 4/20 and Type 3 sub-base as open-graded alternatives. The total depth depends on the ground, the traffic and how much water the pavement has to hold, so it is a design figure rather than a rule of thumb. The defaults of 14 in and 350 mm are starting points to replace with your designer's or supplier's figure.

A worked example of the stone

For 430 ft² at 14 in deep: the bedding is 430 × 2 ÷ 12 = 71.7 ft³, or 2.65 yd³; the base is 143.3 ft³, or 5.31 yd³; and the 8 in of sub-base is 286.7 ft³, or 10.62 yd³. The total is 501.7 ft³, or 18.58 yd³. For 40 m² at 350 mm: 2 m³ of bedding, 4 m³ of base and 8 m³ of sub-base, 14 m³ in all.

Stone is usually sold by weight. The calculator uses 1.43 t/m³, the figure one US supplier gives for #57 crushed granite (2,410 lb/yd³, or 1.21 tons/yd³), so 18.58 yd³ is about 22.4 tons and 14 m³ is about 20 tonnes. The calculator applies that one figure to all three layers, although the finer bedding stone and the coarser sub-base stone each weigh a little differently, so ask your supplier for the figure for each grade you are buying.

The same stone stores water. At 40% voids, 14 m³ of stone holds about 14 × 0.4 = 5.6 m³, or 5,600 litres, which is 140 mm of rain over the 40 m². In US units, 14 in of stone at 40% voids holds 5.6 in of rain, about 1,500 gallons over 430 ft².

Planning and permits

In England, since October 2008, paving more than 5 m² of a house's front garden has counted as permitted development only if the surface is permeable or drains to a permeable area such as a lawn, border or rain garden, as the government guidance explains. Elsewhere, stormwater, driveway and curb-cut rules are set locally, so check with your local authority before starting.

What this tool does not do

It does not design the pavement depth, check the soil's infiltration rate, count edge restraints or joint stone, or price the stone. For a driveway that must hold a set amount of rain, or on clay or other slow-draining ground, the depth and any underdrain need a designer.

Using this calculator with other BuildMetricLab tools

The gravel calculator and sand calculator help with loose materials, the block paver calculator covers conventional block paving, and the dry well calculator sizes a soakaway for roof water. All BuildMetricLab tools run in your browser with no sign-up, and every formula is shown on the page so the maths can be checked.