Rebar Weight: How to Calculate It and Convert Tonnes to Metres

Rebar is designed in metres but sold and shipped by the tonne, so converting between the two comes up on almost every purchase. Get the maths wrong and the site ends up with surplus steel or, on the contrary, not enough to finish the pour. The weight per metre for every diameter is listed on the “Rebar” category page; here we look at how the mass is calculated, how to convert tonnes into metres and bars, and how to be confident in the quality of the steel.

How rebar weight is calculated

The mass of one linear metre depends only on the bar diameter and the density of steel. The density of reinforcing steel is constant at 7,850 kg per cubic metre. From there it is simple geometry: take the cross-sectional area of the bar and multiply it by the density.

The formula is: m = (π × d² / 4) × ρ

where m is the mass of one metre, d is the diameter in metres and ρ is the density of steel (7,850 kg/m³). Insert the diameter, multiply, and you get the weight of one metre. In practice the formula is rarely needed, since ready values are already compiled on the category page, but it explains where those figures come from.

One note on measurement. For ribbed rebar the mass is calculated from the nominal diameter, not from the tips of the ribs. So calipers placed across the ribs will always read slightly above nominal, and that is nothing to worry about.

How to convert tonnes to metres and back

The task runs both ways. From tonnes to metres: divide the batch weight by the weight of one metre of the required diameter. From metres to tonnes: multiply the total length by the weight per metre. It helps to keep the per-metre values from the category page at hand.

Bars are counted separately. To know how many will arrive, divide the total length by the length of a single bar. Medium and large diameters are supplied as fixed-length bars, while thin rebar often comes in coils, so the delivery form and length are worth confirming in advance: both the cutting plan and the transport loading depend on it.

Length tolerances and grade confirmation

The calculated weight is called nominal. Real rolled steel differs slightly from the ideal, and EN 10080 allows for that. With BPM rebar, the deviation most often falls on length, and it works in the client's favour: a bar with a nominal length of 12 metres may in fact measure between 12.01 and 12.08 metres. In other words, you receive slightly more steel, not less.

Nothing prevents you from checking a delivery, and a reliable supplier has no reason to fear it. BPM offices keep calibrated micrometers, and the actual diameter can be measured right at dispatch, including for steel bought elsewhere.

The rebar grade is confirmed by documentation. The steel is produced to EN 10080 in grades B500B and B500C, and compliance is confirmed annually by a Technical Assessment (Evaluare tehnică). B500C offers higher ductility and is specified where the design calls for it, including seismic loads, which is highly relevant for Moldova.

A quick calculation for your order

To estimate a purchase without manual arithmetic, it is easier to put the items together in the order calculator on the BPM website: choose the diameter and quantity, and the weight of the order is calculated automatically. For a complex project with several diameters, it is simpler to send the specification to a manager and ask for a full calculation. It is free and faster than adding everything up by hand.