Plan masonry window sills with confident material totals. Choose block size, width, and fill options. Get quantities, costs, and downloads in one clean form.
| Scenario | Sills | Length (m) | Courses | Blocks | Mortar (m³) | Grout (m³) | Rebar (m) |
|---|---|---|---|---|---|---|---|
| Window sill (small) | 2 | 1.20 | 1 | 7 | 0.0063 | 0.0000 | 0.00 |
| Door threshold sill | 1 | 0.95 | 1 | 4 | 0.0027 | 0.0099 | 2.50 |
| Long opening sill | 1 | 2.40 | 2 | 13 | 0.0134 | 0.0641 | 5.60 |
A block sill is a short run of masonry units built below a window, door, or ventilation opening. Although the length is usually modest, the work is detail-heavy: the first course often needs accurate leveling, joints are highly visible, and reinforcement or grouting may be specified for durability. A reliable takeoff helps you order the right number of units and avoid delays caused by missing blocks, underestimating mortar, or forgetting reinforcement.
This calculator estimates four practical quantities: blocks, mortar, grout, and rebar. Blocks are based on a modular length (block length plus joint thickness). The calculator rounds up to ensure coverage, then applies a waste allowance. If the covered length is slightly longer than the clear opening, that difference represents the typical on-site adjustment—either a cut unit, a closure piece, or a minor joint tweak. Use the overrun value to anticipate where the cut will land and to plan neat reveals.
Mortar is estimated from bed joints (continuous along the sill) and head joints (between adjacent blocks). Real consumption varies with unit texture, joint tooling, and leveling needs, so treat the result as a planning quantity. If your sill detail calls for a thicker bedding layer, if you are using open-ended units, or if you expect heavy trimming, increase the joint thickness or waste percentage to match your practice. For grouted sills, grout volume is calculated from the gross unit volume multiplied by the void ratio and the chosen fill percentage.
Reinforcement can be enabled to suit common details such as crack control, small lintel action, or ties into adjacent masonry. The rebar length is calculated per bar as the clear sill length plus embedment on both sides. Embedment is design-specific, so keep it consistent with drawings and substrate requirements. When costing is enabled, the calculator adds up material costs using your unit rates to provide a quick, transparent estimate.
Example inputs and outputs are shown below. These values also appear in the example table above.
| Item | Value |
|---|---|
| Scenario | Door threshold sill |
| Clear sill length | 0.95 m |
| Sills | 1 |
| Courses | 1 |
| Total blocks (incl. waste) | 4 pcs |
| Mortar volume | 0.0027 m³ |
| Grout volume | 0.0099 m³ |
| Rebar length | 2.50 m |
Use this worked example to sense-check your own project. If the opening has returns, exposed faces, or a tight tolerance on finishes, plan for extra cutting and handling. If the specification requires full grouting, set fill percent to 100 and confirm the void ratio from your unit supplier.
It is the estimated number of blocks needed to span one row across the sill length, assuming a standard joint between blocks. The calculator rounds up so you have enough units to complete the course.
Because blocks are modular, the final run often needs a cut block or a slightly adjusted joint. The covered length and overrun indicate the amount of adjustment you should expect on site.
For straightforward work, 3–5% is typical. For many cuts, difficult access, or frequent handling, 7–10% is safer. Increase waste when breakage risk is higher or when matching strict finish lines.
No. It is a practical estimate based on bed and head joints. Mortar use varies with workmanship, unit texture, joint tooling, and leveling needs. Use it for planning, then refine with site history.
Void ratio is the fraction of a block’s gross volume that is hollow. It directly affects grout volume. If the manufacturer data is unavailable, start with 0.30–0.40 for typical hollow units and adjust later.
Include rebar when drawings show reinforcement, when the sill provides crack control, or when it acts as a small lintel. Always follow the specified bar size, spacing, and embedment from the design notes.
Yes. After you calculate, use CSV for spreadsheets and PDF for a quick print-friendly summary. Exports always use your most recent calculation, so re-run the form whenever you change inputs.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.