Mortar calculator
This turns a unit count into a bag count using the yield you set. Real coverage swings with the joint, the mix and the weather, so keep a spare bag rather than trusting the tally exactly. It is a materials estimate, not a mix design or a structural spec.
Pick whether you are laying block or brick, enter how many and how many one bag lays, and the tool divides the count by the yield, adds a waste margin and rounds up to whole bags of mortar mix.
Worked example
Laying 150 blocks, with a bag good for about 13 blocks: 150 ÷ 13 ≈ 11.5 bags. Add 10% for waste and round up and you would buy 13 bags. Switch to brick at, say, 35 per bag and 600 bricks would need 600 ÷ 35 ≈ 18 bags before waste.
The formula
bags = (units × (1 + waste)) ÷ units per bag, rounded up
The unit count comes from your block or brick tally; the units-per-bag figure is how far one bag stretches with your joint. Dividing gives a fractional bag count, the waste factor absorbs droppings and a stiffened board, and rounding up leaves you with whole bags to carry.
What this tool assumes
Only the yield you type, which is why the tool asks for it rather than guessing. Bag size, material, yield and waste are all set on the page, so nothing about your mortar is fixed behind the scenes.
Questions
- How many bags of mortar do I need?
- Divide your block or brick count by how many one bag lays, then round up. A bag of mortar mix lays roughly a dozen standard blocks or around thirty-odd bricks, though both depend on the bag size and the joint. Enter your count and the units-per-bag figure above and the bag tally, with a little waste, appears at once.
- How many blocks does a bag of mortar lay?
- For a typical 80 lb bag of mortar mix, on the order of twelve to fourteen standard blocks with a normal joint — a smaller metric bag lays fewer. It is the editable default here rather than a fixed figure, because a wider joint, a rougher block or a leaner mix all change it. Set it to what your supplier quotes.
- Does the joint size matter?
- A great deal. A thicker bed and head joint uses more mortar per unit, so fewer units come off each bag. That is exactly why the units-per-bag figure is a field you set rather than a number baked in — dial it down for a fat joint, up for a tight one, and the bag count follows.
- Should I round bags up?
- Always, and the tool does. You buy whole bags, running out mid-course means a cold joint, and mixing a fresh bag late in the day is a nuisance. A spare bag on the pallet is cheap insurance against stopping the wall short of the top.
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