How Many CFT is One Bag Of Cement (50kg)?

Cement bag volume by size

Cement bag weight isn’t the same everywhere. Here’s the cft and m³ figure for every common size, all worked from the same 1,440 kg/m³ bulk density.

The rest of this article uses the 50 kg bag, since it’s the standard size most people mean when they ask this question. If your bag is a different weight, the printed weight is what matters — swap it into the formula below.

Why the exact figure matters

This number gets used in two very different ways, and mixing them up is the single biggest source of error. Get it wrong on a small repair and you’re a bag short. Get it wrong on a full slab or column pour, and the shortfall can be dozens of bags — enough to stall the pour mid-way and leave a cold joint where the old and new concrete never fully bond.

It also matters for costing. Suppliers price cement by the bag or by the tonne, but engineers and masons think in cft and m³. If your conversion is off by even the 2% gap between 1.226 and 1.25 cft, a large order can be short or long by several bags’ worth of material.

Bag volume vs. cement needed for a mix — they’re not the same question

This is where most confusion happens. “1 bag = 1.226 cft” tells you how much space one bag of loose cement powder takes up. It does not tell you how many bags go into a cubic foot of finished concrete — that depends on your mix ratio, because concrete is cement plus sand plus aggregate, not cement alone.

Keep these separate. The first is a unit conversion. The second is a mix-design calculation that uses the first number as one of its inputs — see the worked examples further down.

The simple formula

For a single bag’s own volume:

For a 50 kg bag: 50 ÷ 1440 × 35.3147 = 1.226 cft.

To go from a target cft volume to a bag count:

Always round up. A partial bag still has to be opened.

What you need before you calculate

• Bag weightPrinted on the bag — usually 50 kg, but confirm it.
• Are you converting a bag, or a mix?A straight cft-per-bag conversion needs nothing else. A concrete or mortar quantity needs the mix ratio too.
• Mix ratioe.g. 1:2:4 for general RCC work, 1:4:8 for lean PCC, 1:4 or 1:6 for mortar — printed on your design spec or chosen for the job type.
• Dry volume factorWet, compacted concrete shrinks from its dry, loose ingredient volume. Standard factor is 1.54 for concrete, roughly 1.27–1.33 for plaster and mortar.

Worked examples

Three versions of this question, from simplest to the one that actually matters on a real job.

Example 1 — How many cft is 15 bags of cement?

Example 2 — I need 20 cft of loose cement. How many bags?

Example 3 — Bags needed for 100 cft of 1:2:4 concrete

This third example is the calculation people usually actually need — not “what’s the volume of a bag,” but “how many bags does my job need.” For plastering or mortar, swap the 1.54 dry volume factor for 1.27–1.33 and the ratio for your mortar mix (e.g. 1:4 or 1:6).

Don’t skip the dry volume factor

Wet, compacted concrete or mortar occupies less space than the loose, dry materials that go into it — air gaps between dry particles close up during mixing and compaction. Skip this step and you’ll under-order cement by a third or more.

Multiply your finished (wet) volume by this factor before splitting it across the mix ratio, as shown in Example 3 above.

Quick cross-reference: bags per m³ of finished mix

If you don’t want to run the full dry-volume calculation every time, these are the commonly used thumb-rule figures for 50 kg bags. Treat them as estimates — for a precise take-off, use the full formula above.

When to switch from bagged to bulk cement

For most home and small commercial jobs, bagged cement is the practical choice — easy to store, easy to move in small batches, and you only open what you use. Once a project’s total requirement climbs past roughly 300–400 bags, bulk (silo-delivered) cement usually starts winning on cost per tonne and on labour, since you’re no longer hand-carrying every 50 kg sack to the mixer.

Below that threshold, stick with bags and buy in rounds that match your daily pour capacity — cement stored longer than about three months from its manufacture date starts losing strength-giving reactivity, so over-ordering bagged cement for a slow job wastes money too.

Quick reference: cft to bags (50 kg)

This table converts a raw cft volume of loose cement into bags — it’s not a mix-ratio calculation. For “how many bags for my slab/footing,” use the calculator above or Example 3’s method.

7 mistakes that throw off the calculation

• Treating 1.25 cft as exactIt’s a rounded rule of thumb, about 2% higher than the precise 1.226 cft figure. Fine for a handful of bags, risky on a large order.
• Confusing bag volume with mix bag count“1 bag = 1.226 cft” and “how many bags per m³ of concrete” are different calculations. Don’t substitute one for the other.
• Forgetting the dry volume factorSkipping the 1.54 (or 1.27–1.33) multiplier under-orders cement by a third or more.
• Mixing bag sizes without reconvertingA 50 kg and a 94 lb bag have different volumes. Recheck the printed weight before applying any formula.
• Rounding the bag count down17.94 bags means 18 bags. Concrete doesn’t stretch to cover a missing fraction.
• Ignoring density variance between brandsSome references use 1,400–1,500 kg/m³ instead of 1,440. The gap is small but explains why sources don’t always agree exactly.
• Assuming a caked bag is full qualityA bag that’s absorbed moisture still weighs close to 50 kg, but its usable reactivity is lower. Don’t use it for structural work.

What can change the actual figure

• Cement brand and typeOPC vs. blended cements like PPC can have slightly different bulk densities, shifting the cft-per-bag figure a little.
• Compaction and settlingCement settles during transport and storage; a freshly opened bag can measure slightly differently than the theoretical figure.
• Moisture exposureA bag that’s absorbed humidity weighs the same but has reduced usable reactivity — the “volume” figure stops being meaningful for structural work.
• Which density figure your source uses1,440 kg/m³ is standard, but some references use 1,400–1,500 kg/m³, producing a marginally different result.

Calculate it in five steps

1. Confirm your bag weight. Check the printed figure — don’t assume 50 kg.
2. Get the bag’s volume. Weight ÷ 1440 × 35.3147 = cft per bag (1.226 for 50 kg).
3. Decide what you’re actually solving. A straight bag-to-cft conversion, or a mix-ratio bag count?
4. If it’s a mix, multiply the wet volume by the dry volume factor, then split by the ratio.
5. Divide by 1.226 and round up. Never round a bag count down.

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