What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Statesyours
Same word, different thingType N / S / M / O
also Type S mortar, ASTM mortar types
Letters name a proportion and a strength, with M the strongest and O the weakest. "Type M" has nothing to do with a European M-class number.
Canada
Same word, different thingType N / S / M / O
also CSA mortar types
The same letters as the United States, specified to the Canadian standard — so the letter is familiar and the governing document is not the same one.
United Kingdom
Same word, different thingM-class (M2, M4, M6, M12)
also designation (i) to (iv), mortar class
The M number is the compressive strength in N/mm², NOT a letter grade — and the older British designations (i) to (iv) run the opposite way, with (i) the strongest.
Australia
Same word, different thingM1 to M4
also mortar class
Australian classes run M1 to M4 with M4 the strongest, which is a different numbering again from both the European strength classes and the American letters.
The governing standard, by market
United States
ASTM C270
Mortar for unit masonry — the Type N/S/M/O specification.
United Kingdom
BS EN 998-2
Masonry mortar — including the M-class strength designation.
Australia
AS 3700
Masonry structures, including mortar classes.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Is Type M mortar the same as M4 mortar?
- No, and the collision is close to the worst possible one, because both look like a strength grade and they come from unrelated systems. American Type M is a letter naming a mix proportion and a minimum compressive strength, and it is the strongest of the four letters. A British or European M4 is a strength CLASS whose number is the compressive strength in newtons per square millimetre, so the number is the property directly. A specification reading "Type M" and one reading "M4" are written in different languages, and picking a mortar by matching the letter M gets a considerably stronger mortar than intended — which, counter-intuitively, is usually the wrong direction.
- Why is a stronger mortar not automatically better?
- Because the mortar is meant to be weaker than the units it joins, and a mortar that is too strong moves the failure. Masonry moves — thermally, with moisture, with settlement — and a comparatively weak mortar accommodates that movement by cracking in the joints, where cracks are fine, distributed and repointable. A mortar stronger than the brick concentrates the movement into the units instead, so the cracks run through the bricks and the wall is no longer repairable by raking out and repointing. That is why specifications call for the weakest mortar consistent with the exposure and the loading, why lime mortars are specified for older soft masonry, and why repointing in a strong cement mortar damages historic brickwork.
- How do the older British designations relate to the M-classes?
- They run in the opposite direction, which is a second trap in the same field. The older British designations (i), (ii), (iii) and (iv) number the mixes with (i) the STRONGEST and (iv) the weakest, so the number rises as the strength falls. The European M-classes number by strength in N/mm², so the number rises as the strength rises. Both appear in British documents — older specifications and many trade references still use the designations, and current standards use the classes — so a document mixing them is not unusual and reading a number without knowing which system it belongs to inverts the intent. The correct approach is to read the mix proportions or the stated strength rather than the label.