About This Calculator
This tool helps heritage masonry contractors, conservation builders, and knowledgeable DIYers estimate the quantity of Natural Hydraulic Lime (NHL) binder, sand, and total material cost needed for a repointing project. It covers the full workflow: from wall measurement and joint geometry through exposure-guided NHL grade selection to the critical RBD-based volume-to-weight conversion that separates a reliable estimate from a rough guess.
Unlike generic mortar calculators, this tool speaks the dialect of traditional lime work — NHL grades, volumetric mix ratios, relative bulk density, and the "mortar must be weaker than the masonry" principle central to conservation practice.
How to Use This Calculator
- Enter wall area — length × height, then deduct window and door openings.
- Set joint geometry — choose your masonry preset or enter custom unit dimensions. The joint fraction (% of face area that is mortar joint) is auto-calculated but can be overridden, especially for irregular stone.
- Set rake-out depth — the depth to which old mortar will be removed. At least 2–2.5 times the joint width, minimum 15 mm.
- Choose NHL grade and check exposure guidance — the selector updates the recommended grade based on exposure; you can override it by clicking a grade card.
- Confirm mix ratio and binder data — the volumetric lime:sand ratio, dry-to-wet factor, and each material's bulk density (check your NHL bag label). These convert volume to weight accurately.
- Add optional prices to get a material cost breakdown.
Results update live as you type. Use Print / Save as PDF for a clean job-sheet, or Download CSV to import into a spreadsheet.
Choosing the Right NHL Grade
The overriding rule of heritage masonry is that mortar must be softer and weaker than the masonry units it binds. Too-strong mortar traps moisture and transfers stress into the bricks or stone rather than accommodating movement in the joint — leading to spalling and cracking that can be irreversible.
- NHL 2 (Feebly hydraulic) — Very slow-setting; maximum breathability and flexibility. Use for sheltered internal work, very soft historic brick or friable sandstone, and the most delicate Grade I listed structures. Requires protection from frost for up to 14 days.
- NHL 3.5 (Moderately hydraulic) — The workhorse of UK heritage repointing. Suitable for the vast majority of Georgian, Victorian, and Edwardian brickwork and most coursed stonework. Balances early strength with good vapour permeability. Frost protection needed for ~10 days.
- NHL 5 (Eminently hydraulic) — Fastest-setting and strongest. Use only where exposure demands it: exposed chimney stacks, parapets, copings, below-DPC work, coastal and upland sites. Even NHL 5 remains far more breathable and flexible than Portland cement. Only 1 day frost protection typically required.
Why Volumetric Mix Ratios Must Be Converted via RBD
NHL mix ratios (e.g., 1 part lime : 2.5 parts sand) are specified by volume. But you buy materials by weight (kg bags, tonne bags). The conversion requires each material's Relative Bulk Density (RBD, in kg/litre), which varies between NHL grades and brands — a 25 kg bag of NHL 2 (RBD ≈ 0.50 kg/L) occupies more volume than a 25 kg bag of NHL 5 (RBD ≈ 0.55 kg/L). This calculator does that conversion for you using the RBD values from your bag label (editable in Step 4).
Typical NHL Mix Ratios by Grade
- NHL 2: 1 part lime : 2.5–3 parts sand (by volume)
- NHL 3.5: 1 part lime : 2.5–3 parts sand (by volume)
- NHL 5: 1 part lime : 2–2.5 parts sand (by volume)
For historic work matching an existing mortar, laboratory mortar analysis is the gold standard. For new specification work, select based on masonry hardness and exposure as above.
Frequently Asked Questions
What NHL grade should I use for repointing historic brickwork?
The right NHL grade depends on exposure and masonry hardness. NHL 2 suits sheltered locations and very soft, fragile historic brick or stone. NHL 3.5 is the general-purpose choice for most UK and European historic buildings. NHL 5 is reserved for exposed locations such as chimneys, parapets, and coastal walls. The guiding principle is that mortar must always be softer and weaker than the masonry units it binds.
How deep should I rake out joints before repointing with lime mortar?
Conservation guidance recommends raking out to a minimum depth of 2 to 2.5 times the joint width, with an absolute minimum of 15 mm. Raking should be done by hand using plugging chisels; power tools risk chipping masonry arises. Always pre-wet the raked joint before applying fresh mortar to prevent rapid suction from the masonry drying the mortar too quickly and weakening the bond.
Why do NHL lime mix ratios need to be measured by volume, not weight?
Because the bulk density of NHL powders varies between grades and brands, measuring by weight instead of volume produces a mortar that is either too lean or too rich. For accurate batch production, convert the volumetric ratio to weight using the binder's Relative Bulk Density (RBD, in kg/litre), which this calculator does automatically. Check the figure on your bag label and enter it in Step 4.
What is the dry-to-wet volume factor for lime mortar?
A factor of 1.33 is commonly used, meaning approximately 1.33 parts of dry mixed ingredients produce 1 part of finished wet mortar, because the binder fills the voids between sand grains rather than adding to total volume. This calculator uses 1.33 by default but the value is editable.
How much waste should I allow for lime repointing?
A 10–15% waste allowance is standard for typical brickwork repointing, rising to 20% or more for irregular stone joints or difficult access. It is always better to order a little more than needed — running short risks a visible colour mismatch where two batches meet.
Can I use this calculator for lime putty mortar?
No. This calculator is designed for NHL (Natural Hydraulic Lime) powder mortars only. Lime putty is a pre-hydrated, paste-form non-hydraulic lime with very different density and curing characteristics — it sets entirely by carbonation rather than hydraulic hardening. NHL volume-to-weight conversions should not be applied to lime putty.