Running out of backfill compound halfway through a pit means a half-filled bore and a poor reading. Ordering far too much ties up money and storage. Here is how to estimate what each chemical earth pit needs, and how to turn that into an order for a whole project.
What decides the quantity
- The bore: its diameter and depth set the volume to be filled.
- The electrode: its own volume takes up part of the bore.
- How the compound is placed: dry, or mixed as a slurry with water, and how firmly it is compacted.
- The soil: loose or fissured ground can take more than the calculation suggests.
- The design: some specifications call for compound only around the electrode, others for a full bore of compound.
Step 1: work out the volume to fill
Treat the bore as a cylinder and subtract the electrode:
- Bore volume = 3.14 × (bore diameter ÷ 2)² × filled depth.
- Electrode volume = 3.14 × (electrode diameter ÷ 2)² × length in the bore.
- Volume to fill = bore volume − electrode volume.
Use metres throughout and the answer is in cubic metres. A wider bore increases the volume quickly, because the diameter is squared.
Step 2: turn volume into bags
How much volume one bag fills depends on the grade and on whether it is placed dry or as a slurry. Ask us for the coverage of the grade you are using, or check it on site by filling the first pit and counting the bags. Then:
- Bags per pit = volume to fill ÷ volume one bag fills, rounded up.
- Bags for the project = bags per pit × number of pits.
- Add a small margin, typically one spare bag for every ten pits, for losses and uneven bores.
Bag sizes
| Compound | Bag | Typical use |
|---|---|---|
| BFC Commercial | 25 kg | Everyday installations |
| BFC Standard (Standard, Advance, Premium, Elite) | 25 kg | Projects with a specified resistance, dry or difficult soil |
| BFC Solar Earthing Grade | 15 kg | Rooftop and ground-mounted solar |
Use the calculator for the project total
Once you know the bags per pit, our material calculator works out the total bags and weight for any number of pits, for both 25 kg and 15 kg bags. It is handy for checking a BOQ before you send it out.
Placing the compound properly
- Centre the electrode in the bore before filling, so the compound surrounds it evenly.
- Fill in layers and compact each one, or pour as a slurry if the product instructions say so. Air pockets raise the resistance.
- Fill the full specified depth. A pit filled only at the bottom will not give the designed reading.
- Finish with soil, then fit the inspection chamber and pit cover.
Choosing the grade
Higher grades hold moisture for longer and keep resistance lower through dry months. For ordinary soil, Commercial or Standard is usually enough. For sandy, rocky or very dry ground, or where the specification sets a low resistance target, choose a higher grade. Our earlier guide on choosing electrode size explains how soil and target fit together.
Frequently asked questions
Can salt and charcoal be used instead of backfill compound?
Salt and charcoal belong to traditional pipe and plate earthing. They work while the pit is watered, but the salt washes out over time and can speed up corrosion. Backfill compound is designed to stay conductive without that upkeep.
Should backfill compound be placed dry or mixed with water?
Follow the instructions for the product and grade you are using. Either way, the compound must surround the electrode evenly with no air pockets.
How should backfill compound be stored?
Keep the bags sealed, dry and off the ground until they are used. Compound that has absorbed moisture in storage is harder to place evenly.
Tell us the bore size, electrode, soil and number of pits, and we will work out the quantity and recommend the grade. Ask for a quantity estimate.