Short answer: a "400 × 200" CHB is not 400 × 200. That is the nominal size — the space the block occupies in a finished wall, mortar joint included. The block itself is one joint smaller, about 390 × 190 mm, and the standard lets the delivered block vary from that by ±3.2 mm. Everything else on this page follows from that one distinction, including the number most people actually need: what the wall weighs.
Blocks are sold by the module, laid by the joint, and designed by the unit. Suppliers quote 400 × 200 because that is what a block occupies; your tape measure says 390 × 190 because that is what a block is. Both are correct. Only one of them belongs in a take-off.
Masonry uses three dimensions for the same block, and mixing them up is where quantity errors and site arguments start.
The planning module. What the block plus one mortar joint occupies in the wall. This is what suppliers advertise and what you use to lay out openings and set out courses.
Nominal minus one mortar joint — 10 mm in Philippine practice. This is the size the block is made to, and the size a delivery is judged against.
What comes off the truck. ASTM C90 §6.1 permits no overall dimension to differ from specified by more than ±3.2 mm (±1/8 in).
The same subtraction applies to thickness. A "6-inch" block is nominally 150 mm wide and specified at about 140 mm — which is why a plastered 6-inch wall finishes near 175 mm, not 190 mm, and why door jambs ordered off the nominal figure come back wrong.
| Trade name | Nominal (W × H × L) | Specified (approx.) | Typical use |
|---|---|---|---|
| 4-inch | 100 × 200 × 400 mm | 90 × 190 × 390 | Interior partitions, non-loadbearing infill, light fences |
| 5-inch | 125 × 200 × 400 mm | 115 × 190 × 390 | Less common; regional availability varies |
| 6-inch | 150 × 200 × 400 mm | 140 × 190 × 390 | Exterior walls, perimeter fences, loadbearing |
| 8-inch | 200 × 200 × 400 mm | 190 × 190 × 390 | Retaining walls, tall fences, commercial loadbearing |
Where the 12.5 Blocks per Square Metre Comes From
A nominal face of 400 × 200 mm is 0.08 m². One square metre divided by 0.08 is exactly 12.5 blocks — and that is why the figure is a clean number rather than a rule of thumb.
It does not change with wall thickness. A 4-inch and an 8-inch wall both take 12.5 blocks per square metre, because only the width changes; the face stays 400 × 200. Order 13–13.5 per m² to cover breakage, cutting at openings, and the blocks you reject at the gate. Full take-off in our CHB quantity estimator.
Converts a block size into the numbers you actually need: specified dimensions and tolerance, the face shell minimum the standard requires, blocks and mortar per square metre, and — the one nobody publishes — the wall dead load in kPa and as a line load on the beam below.
A lot of Philippine specifications and price lists still cite PNS 16:1984. That standard has been superseded.
In November 2019 the DTI Bureau of Product Standards adopted three international standards as Philippine National Standards, and they cancelled and replaced PNS 16:1984:
The practical consequence is the strength requirement. Under C90 the minimum net area compressive strength is 13.8 MPa (2,000 psi) as the average of three units, with no single unit below 12.4 MPa (1,800 psi) — the same for lightweight, medium weight and normal weight units.
Two Words That Change What a Test Report Means
"Net area." The strength is measured on the solid material only, not on the full rectangle of the block. A block that is mostly void can post a respectable net-area number while contributing much less to the wall than you assumed. Net area is why face shell thickness is a strength question, not a cosmetic one.
"Average of three." One passing block proves nothing about a delivery. C90 sets both an average of three units and an individual-unit floor, so a report showing a single result is not evidence of compliance.
Undersized length and height are easy to spot with a tape. The face shell is where a block quietly loses strength, because you have to look at the end of it to see the problem, and by then it is on a pallet in your yard.
| Nominal width | Min. face shell | Min. web thickness | Min. normalized web area |
|---|---|---|---|
| 76 mm and 102 mm (3" & 4") | 19 mm | 19 mm | 45,140 mm²/m² |
| 152 mm (6") | 25 mm | 19 mm | 45,140 mm²/m² |
| 203 mm (8") and greater | 32 mm | 19 mm | 45,140 mm²/m² |
A thin face shell costs you twice: less net area carrying load, and less sound material for plaster to bond to. Hairline map-cracking across a newly plastered wall is very often a block problem wearing a plaster problem's clothes. Our plastering cost guide covers the finishing side.
Do this at the gate, before the truck leaves, on every delivery — not just the first one. Quality drifts between batches.
Dimensional tolerance (±3.2 mm), face shell and web minimums, and the 13.8 / 12.4 MPa net-area strength requirement are from ASTM C90, adopted in the Philippines as PNS ASTM C90:2019. The adoption and the cancellation of PNS 16:1984 are recorded in the DTI Bureau of Product Standards announcement of November 2019. The nominal-minus-one-joint convention is the standard masonry definition of specified dimensions. Wall unit weights are the NSCP values in our dead and live loads reference. Prices referenced elsewhere on the site are AEDO 2026 planning ranges, not quotations.
What is the standard CHB size in the Philippines?
The standard Philippine concrete hollow block has a nominal face of 400 by 200 millimetres, in nominal widths of 100, 150 and 200 millimetres, commonly called 4, 6 and 8 inch. Those are nominal dimensions, meaning the planning module including the mortar joint. The block itself is made smaller by the thickness of one joint, so with the 10 millimetre joint used in Philippine practice a nominal 400 by 200 by 150 block is specified at about 390 by 190 by 140 millimetres. Under PNS ASTM C90:2019 the manufactured block may then vary from that specified size by no more than plus or minus 3.2 millimetres on any overall dimension.
Why is a 400 x 200 hollow block actually 390 x 190?
Because masonry is designed around a module, not around the block. The nominal dimension is the space one block occupies in a finished wall, which includes its mortar joint. Subtract one joint and you get the specified dimension of the unit itself. Philippine practice uses a 10 millimetre joint, so 400 becomes 390 and 200 becomes 190. This is what makes the arithmetic work: 400 by 200 millimetres is exactly one twelve-point-five-thousandth of a square metre, which is why a square metre of wall takes 12.5 blocks regardless of thickness.
What standard covers concrete hollow blocks in the Philippines?
PNS ASTM C90:2019 with Amendment 1:2019 for loadbearing units and PNS ASTM C129:2019 with Amendment 1:2019 for nonloadbearing units, with PNS ASTM C140/C140M:2019 covering sampling and test methods. The Department of Trade and Industry Bureau of Product Standards adopted these in November 2019, and they cancelled and replaced the older PNS 16:1984 Specification for Concrete Hollow Blocks. Any specification or price list still citing PNS 16 is referencing a standard that has been superseded for more than six years.
What is the minimum compressive strength of CHB?
For loadbearing units under ASTM C90 the minimum net area compressive strength is 13.8 megapascals, about 2,000 pounds per square inch, as the average of three units, with no individual unit below 12.4 megapascals or 1,800 pounds per square inch. The requirement is the same for lightweight, medium weight and normal weight units. Two details matter when comparing test reports: the strength is measured on the net area, meaning the solid material only rather than the full rectangle of the block, and it is an average of three units, so a single passing block proves nothing about a delivery.
How many CHB are needed per square metre?
Exactly 12.5 blocks per square metre in theory, because the 400 by 200 millimetre nominal face means each block plus its mortar joint occupies 0.08 square metres. In practice order about 13 to 13.5 per square metre to cover breakage, cutting at openings and corners, and the blocks rejected on delivery. The figure does not change with wall thickness, since only the width of the block changes and the face stays 400 by 200.
How do I check whether delivered blocks are undersized?
Measure ten blocks from different pallets with a steel rule or calliper and compare against the specified dimension, not the nominal one. For a nominal 400 by 200 unit the specified size is about 390 by 190 and the permitted variation under ASTM C90 is plus or minus 3.2 millimetres, so anything below roughly 387 by 187 is outside tolerance. Then measure the face shell, which is where cheap blocks are thinnest: the minimum is 19 millimetres for 100 millimetre units, 25 millimetres for 150 millimetre units, and 32 millimetres for 200 millimetre units and wider. A thin face shell reduces both strength and the surface available to bond plaster.
Block counts are the easy part. An itemised BOQ takes the walls off your actual drawings and prices the cement, sand, rebar and labour with them — so every bidder prices the same list instead of their own assumption.