Hempcrete Calculator
Work out how much hemp hurd and lime binder a hempcrete wall needs. Enter your wall dimensions or the cubic footage directly, and this returns dry material weights plus a mixing water estimate.
How to use it
- Measure the wall you are filling — length and height in feet, thickness in inches. A typical hempcrete wall is cast 10–12 inches thick.
- Enter those three numbers, or skip them and type the cubic footage straight in if you already have it.
- Leave the waste buffer ticked. Hempcrete is cast by hand into shuttering and you will lose some.
- Read off the hurd and binder weights. Treat the water figure as a starting point, not a recipe — see the note underneath.
Hempcrete material calculator
or
Enter either all three dimensions or a cubic footage.
- Hemp hurd—6.5 lb per ft³
- Lime binder—10 lb per ft³
- Water, starting point—adjust on site
- Cast weight, dry—hurd + binder
On the water figure. Hempcrete water demand moves with the moisture already in your hurd, the binder you use and the weather on the day. Mix to the squeeze test instead of to a number: a handful squeezed in your fist should hold its shape and just glisten, without water running out. Start drier than you think and add.
Basis: 6.5 lb of hemp hurd and 10 lb of lime binder per cubic foot — a widely used starting point for cast wall infill. Floors, slabs and roof insulation are mixed at different densities and this calculator does not cover them.
What hempcrete is, and what it is not
Hempcrete is hemp hurd — the woody core of the stalk, left over once the fiber has been stripped — bound with a lime-based binder and water. It is cast wet into shuttering around a frame, or sprayed, and left to set. The lime holds the hurd in place and carbonates slowly over months.
It is not a structural material. Hempcrete has low compressive strength and will not carry a roof. It is infill and insulation, and it needs a structural frame — usually timber — to work around. Any design that treats it as load-bearing is wrong, and that is the single most common misunderstanding about it.
What it does well is manage heat and moisture together. It insulates while remaining vapor-open, so the wall buffers humidity rather than trapping it. That combination is why it behaves differently in practice from a wall insulated to the same nominal R-value with a closed material.
Where it gets used
- Wall infill around a timber frame — the most common use by a wide margin.
- Retrofit on solid walls, cast against existing masonry, where a vapor-open insulation is needed to avoid trapping damp in old fabric.
- Floor build-ups, mixed denser, as an insulating layer under a screed.
- Roof and ceiling insulation, mixed lighter, where the weight can be carried.
- Precast blocks and panels, made off site and laid like masonry.
How it compares
Hempcrete does not win on R-value per inch. Compared against foam boards it is clearly worse on that one number, and honest sources say so. Where it separates is on everything the R-value does not capture — moisture behavior, thermal mass, fire behavior and end-of-life.
| Property | Hempcrete | Rigid foam | Mineral wool |
|---|---|---|---|
| Insulating value per inch | Modest | High | Moderate |
| Vapor-open | Yes | No | Yes |
| Thermal mass | Yes | Negligible | Negligible |
| Structural | No — needs a frame | No | No |
| Installed as | Cast or sprayed wet | Boards | Batts or boards |
| Cure time before enclosing | Weeks to months | None | None |
The cure time is the practical catch that catches people out. A cast hempcrete wall has to dry before it can be closed up, and that timeline depends on thickness, weather and airflow. It is not a material for a rushed program.
Technical data
Published figures vary between binders, hurd sources and mix designs. These are the ranges you will see cited for cast wall-density hempcrete, and they are ranges rather than specifications — your supplier's data sheet governs.
| Dry density, wall mix | About 17–21 lb per ft³ (275–330 kg per m³) |
|---|---|
| Compressive strength | Low — non-structural. Frame carries the load. |
| Vapor permeability | High. The wall breathes. |
| Binder | Lime-based, formulated for hempcrete. Not ordinary Portland cement. |
| Typical wall thickness | 10–12 in for new build wall infill |
On the binder
Use a binder formulated for hempcrete. Portland cement is the wrong material here: it is too rigid and too closed for a hurd matrix, and it undermines the vapor-open behavior that makes the wall worth building. Lime binders sold for hempcrete are usually blends, and each one has its own water demand and cure profile — which is why the water figure above is a starting point rather than a recipe.
Mixing and placing
- Binder and water first. Combine them to a slurry before the hurd goes in, so the binder is already wet when it meets the hurd.
- Add the hurd and mix until coated. You are aiming for every piece lightly covered, not a soup. Over-mixing breaks the hurd down.
- Squeeze test. A handful should hold together and glisten. If water runs out, it is too wet and it will slump and cure slowly.
- Place in lifts and tamp lightly. Firm enough to hold, loose enough to breathe. Compacting hard raises density and defeats the insulation.
- Strike the shuttering early so the face can dry, and keep air moving across the wall while it cures.
Sourcing the materials
We list hemp hurd in several grades — whole hurd for casting, and milled and powdered grades used in other applications. Availability moves with the harvest, so check the current stock position before you plan around a delivery date.
We do not currently stock lime binder. If you are pricing a build and want help sourcing binder alongside hurd, email Shop@AllHemp.com with your cubic footage and location and we will point you at suppliers who serve your area.
Estimation only. This calculator returns an estimate for planning. Real material use varies with your binder, the moisture in your hurd, wall geometry, shuttering, weather and how the mix is placed. AllHemp.com accepts no liability for the outcome of any build. Hempcrete is not a structural material and must be designed around a load-bearing frame — consult a qualified engineer or an experienced hempcrete contractor before building.