Mortar Calculator
How many 80 lb bags of mortar mix a block or brick job takes — from the unit count, using the coverage rates masons estimate with.
Result
| Units to lay | |
| Coverage per bag | |
| Exact bags needed |
Units & conversions used
This calculator works in counts, not dimensions, so no imperial/metric conversion is needed — a block or brick count is the same everywhere. The bag size, however, is a US convention: coverage is stated per 80 lb (≈36 kg) bag of pre-mixed mortar. If you buy 60 lb bags instead, multiply the bag count by 80 ÷ 60 (about 1.33×) for the same coverage.
Formula & source
Block (8×8×16 in): ≈ 35 per bag · Modular brick: ≈ 100 per bag
Source: these per-bag yields are the long-standing estimating rules of thumb published by pre-mix manufacturers and masonry references for an 80 lb bag with standard ⅜ in mortar joints. They are rules of thumb, not guarantees: thick joints, deep struck joints, unit size variation and on-site waste all reduce the real yield, which is why the result is always rounded up to whole bags.
Worked example (you can verify by hand)
Input: 350 concrete blocks (8×8×16 in).
- Coverage: about 35 blocks per 80 lb bag
- Bags: 350 ÷ 35 = 10.0
- Round up to whole bags: 10
Expected output: 10 bags of mortar mix (80 lb).
Limitations — what this cannot do
- Coverage assumes standard ⅜ in joints and nominal unit sizes. Wider joints, larger-than-modular units, or buttering both faces can use noticeably more mortar per unit.
- It counts laying mortar only — not parging, grout fill for reinforced cells, or mortar for repairs, which are estimated separately.
- Pre-mixed bag yields assume you mix to the bag's stated consistency; soupy mortar spreads further but cures weaker. Mortar type (N, S, M) is chosen by structural requirements and code, not by this calculator.
FAQ
How many bags of mortar do I need for 100 blocks?
100 ÷ 35 = 2.86, so 3 bags of 80 lb pre-mix. Buy at least one extra bag if the wall has many cuts or corners — running out mid-wall costs more in lost time than a returned bag.
How much does an 80 lb bag of mortar mix cost?
An 80 lb bag of pre-mixed mortar commonly runs in the $5–$9 range at US home centers (2025–2026), with Type S and specialty mixes at the upper end. Site-mixed mortar (portland cement, lime and sand bought separately) is cheaper per yard of wall but needs on-site proportioning and mixing space.
Why does one bag lay so many more bricks than blocks?
Mostly unit size: a standard block (8×8×16 in) has roughly 3 times the bed-joint area of a modular brick, so it consumes about 3 times the mortar. Both figures assume the same ⅜ in joint.
How long can I work a batch of mortar?
Mixed mortar is typically workable for about 1½–2½ hours depending on temperature; standards such as ASTM C270 practice allow retempering (adding a little water and remixing) within that window to restore workability. After it starts to set, discard it — retempered stiff mortar loses bond strength. Mix half-bags on small jobs to avoid waste.