Groundwork Time Calculator

How the groundwork time calculator works

Groundworks done by hand, clearing a plot or digging out for a base, path or strip foundation, take time in proportion to how much ground there is and how hard it is to work. The calculator takes the area, the task and, for digging, the depth, and turns them into person-hours, elapsed hours for the crew, working days and a labour cost.

The production rates come from the International Labour Organization's Productivity Norms for Labour-Based Construction (ILO/ASIST Technical Brief No. 2), which compiled output from labour-based projects in ten countries and a World Bank study into recommended norms for an eight-hour worker-day. For digging it recommends 5.0 m³ of in-situ soil per worker-day in soft soil, 3.5 in firm, 3.0 in stiff, 2.0 in very stiff and 0.8 in rock. For site clearing it recommends 350 m² a day in light bush that a bushknife will cut, 200 in medium bush that needs a scythe, axe or bowsaw, 100 in dense bush that needs an axe or chainsaw, and 175 m² a day for stripping topsoil, which the brief notes is sometimes called grubbing. Stumps of trees over 20 cm across are priced one by one, not by area.

The formula

For digging, the hours for each square metre are the depth times eight hours, divided by the norm: r = d × 8 ÷ N. For clearing it is simply r = 8 ÷ N. The person-hours are the area times r; the elapsed hours are the person-hours divided by the crew size; working days are elapsed hours divided by the hours worked a day; and the labour cost is the person-hours times the hourly rate. If you already know your own rate, type it into the override box and the norm is ignored.

Worked examples

Digging out a 20 m² base 300 mm deep in medium ground (firm clay) moves 20 × 0.3 = 6 m³. At 3.5 m³ a day that is 6 ÷ 3.5 = 1.71 worker-days, or 13.7 hours for one person, which works out at 0.686 hours per square metre. At £25 an hour the labour is £342.86. The same job in soft soil with two people takes 4.8 hours on site. In US units, 215 ft² dug 12 in deep is 7.96 yd³ and takes one laborer 13.9 hours; at $55 an hour that is $765.36.

Clearing is much quicker per square metre. Two people clearing 400 m² of medium bush at 200 m² each a day need 16 person-hours, a single eight-hour day between them.

Soil classes

The ILO brief classes soil by how it digs. Soft ground, soft clay or very loose sand, can be dug with a shovel or hoe. Medium ground, firm clay or loose sand, can still be dug with a shovel. Hard ground, stiff clay or compact sand, needs a pick or mattock. Very hard ground, very stiff clay or dense sand, needs a crowbar as well as a pick, and rock needs a sledgehammer and chisels. Pick the class you find when you put a spade in, not the one on the geology map; ground that is wet, full of roots or stony digs more slowly than its class suggests.

Where the norms come from, and what to adjust

The norms were set for labour-based road and drainage work, where workers dig in the open with hand tools and throw the spoil to one side. The ILO found that trained workers usually met the task in 70 per cent of the working day and often in half of it, so the figures are achievable rather than generous. Work in a confined trench, next to services or under a building, or spoil that has to be barrowed away, all take longer. The ILO lists loading, haulage and compaction as separate activities, and so does this calculator: add them on, or use a machine. The excavation time calculator covers digging with a mini excavator, which is much faster for anything but small jobs.

Crew size and working days

More people shorten the job in proportion only while each has room to work and somewhere to put the spoil. On a small plot, two or three people are usually the practical limit. Working days assume the hours per day you enter; the norms themselves are per eight-hour day, so a longer day does not guarantee proportionally more output.

Safety in excavations

Trenches collapse without warning, and a cubic metre of soil weighs well over a tonne. In the United States, OSHA's 29 CFR 1926.652 requires a protective system for excavations 5 ft (1.5 m) deep or more unless they are in stable rock. In Great Britain, the HSE's excavation guidance is that no ground can be relied upon to stand unsupported in all circumstances. Locate buried services before breaking ground, keep spoil back from the edge, and check the rules where you are.

Using this with other BuildMetricLab tools

The excavation volume calculator gives the volume of a hole from its dimensions, the soil compaction calculator what the spoil will bulk to in a skip or dumpster and compact back to, and the excavation time calculator the time for machine digging. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.