Concrete Mixer Trailers: Drum Volume, Legal Payload and Cost per m³
By STU Trailers Engineering TeamJuly 30, 20269 min readLast updated July 30, 2026
A 14.7 m³ drum does not mean 14.7 m³ of concrete on every route. Concrete weighs roughly 2.4 tonnes per cubic metre, so the weight limit in your country decides how much of the drum you can legally fill. Here is how to work it out before you buy.
Concrete weighs roughly 2.4 tonnes per cubic metre. That single figure governs everything about mixer trailer specification, because it means a large drum fills up on weight long before it fills up on volume. A 14.7 m³ drum holds around 35 tonnes of concrete when full, and in markets operating to a gross combination limit of 40 tonnes that is far more than the combination can legally carry. Understanding the gap between drum volume and legal volume is the difference between buying the right unit and buying a drum you can never fill.
The short answer
Take your national gross combination weight limit, subtract the tractor unit and the trailer's tare weight, and divide what remains by 2.4. That number is the cubic metres of concrete you can legally deliver per trip. In a market with a gross limit of 40 tonnes that may, for example, be between 9 and 10 m³, depending on actual operating mass and axle limits. In markets with higher limits, or on site work where road limits do not apply, larger drums come into their own. Specify the drum to the limit you actually operate under, not to the largest figure in the catalogue.
Why drum volume and legal volume are different numbers
Geometric drum volume is the total internal space. Rated mixing capacity is the volume of concrete the mixer is designed to mix and transport; it is smaller than the geometric volume. The nominal filling figure in a specification must not be confused with geometric volume. Compare the manufacturer's rated capacity with the payload available for the actual vehicle and route.
Neither figure accounts for road weight limits. That is a separate calculation, and in most European operations it is the binding one. Work it in this order: legal gross combination weight, minus tractor weight, minus trailer tare weight, gives payload in tonnes. Divide by 2.4 and you have your deliverable volume in cubic metres.
If that result is well below your drum's rated capacity, the extra drum volume is carrying air on every journey while costing you tare weight. If the result is close to rated capacity, the specification is matched.
Where mixer trailers beat truck mixers
A truck mixer is a rigid vehicle with the drum permanently mounted. A mixer trailer is a articulated trailer, which means the tractor unit is separable. That distinction drives most of the operating economics.
Volume per trip is higher. Because a articulated trailer distributes weight across more axles than a rigid truck mixer with four axles, it can usually carry more concrete within the same national weight limit. For a batching plant measuring cost per cubic metre delivered, that difference compounds across every load of every day.
Tractor utilisation improves. A truck mixer sits idle whenever the drum is not in use. A tractor unit can drop one mixer trailer and pick up another, or move a different trailer type entirely. Fleets running mixed work often justify mixer trailers on tractor utilisation alone.
Replacement cost separates. When a drum reaches end of life, you replace a trailer rather than a complete vehicle. When a tractor reaches end of life, you replace it without touching the mixing equipment.
Where truck mixers still win is manoeuvrability on constrained sites and short urban delivery cycles. A rigid mixer with four axles turns inside spaces a combination measuring 16.5 metres cannot enter. Most batching plants that run mixer trailers keep some truck mixers for exactly that reason.
Drum drive systems
Drum rotation keeps the concrete mixture homogeneous during transport; it does not stop cement hydration. Follow the specified delivery time and mixing procedure. STU supplies both PTO drive and an independent auxiliary engine.
A PTO drive runs the drum's hydraulic circuit from the tractor. It removes a second engine from the trailer, which means less weight, less fuel and less maintenance. The condition is that every tractor pulling the unit must carry a compatible PTO and hydraulic circuit, and the drum stops turning whenever the trailer is uncoupled. For fleets running fixed tractor and trailer pairings, this is usually the more economical arrangement.
An auxiliary engine mounted on the trailer drives the drum independently, typically a Deutz or equivalent industrial diesel unit. This frees the trailer from any requirement on the tractor, keeps the drum turning while the unit is dropped at a plant or on site, and allows any tractor in the yard to pull it. The additional requirement is a second engine to fuel, service and maintain, plus its weight on the chassis.
The decision is driven by fleet structure rather than by engineering. Dedicated pairings favour PTO. Mixed fleets and operations involving trailer exchanges favour the auxiliary engine.
Water tank and washout
Onboard water is used to wash the chute and drum opening after discharge. Any adjustment to the concrete at delivery must follow the approved mix specification and the supplier's procedure. STU specifies a water tank of 600 litres. The admixture tank of 40 litres is optional on the 12.7 m³ model and standard on the 14.7 m³ model. Confirm the required tank capacities in the order specification.
Choosing insufficient tank capacity is a common and expensive mistake. A driver who runs out of washout water leaves hardened concrete in the chute and around the drum opening, which accumulates load after load, adds dead weight, and eventually requires a chipping operation that takes the unit out of service.
Drum wear and blade life
The inside of a mixer drum is an abrasive environment. Aggregate scours the drum wall and the mixing blades continuously, and worn blades stop moving concrete efficiently, which lengthens mixing time and reduces discharge speed.
Drum shell thickness and blade material are therefore direct operating cost decisions, not construction details. A thicker shell and harder blade material cost more at purchase and last longer under aggregate abrasion. For a plant running multiple loads a day, blade replacement intervals affect availability more than most buyers anticipate at the specification stage.
Discharge, chute reach and site access
Concrete has to arrive where the pour is, not merely at the site gate. Chute length and articulation determine how far from the trailer you can place concrete without a pump. Discharge height determines whether you can load a pump hopper or a skip directly.
Where the pour point is beyond chute reach, a concrete pump is required, which changes the cost of the job. Specifying adequate chute reach for the work you typically do reduces how often that happens.
Approach also matters. Mixer trailers are long, and batching plant yards and construction site approaches are frequently not. Where site access is a recurring constraint, the honest answer is sometimes a truck mixer rather than a larger trailer.
Specifying by the market you operate in
Weight limits vary substantially, and mixer specification should follow them rather than a global standard.
The delivery volume depends on the actual operating mass of the tractor and trailer, including water, fuel and equipment, as well as the applicable gross and axle limits. A figure of 9 to 10 m³ under a gross limit of 40 tonnes is only an example, not a guaranteed capacity for every model. Higher permitted weights may allow more concrete per trip. On closed sites, even where road limits do not apply, the ratings of the drum, chassis, axles and tyres and the site's safety rules still apply.
This is why the same manufacturer offers a range of drum volumes rather than one size. The right drum is the one matched to the legal payload where the unit will work.
What catches operators out
Three things, repeatedly. Buying drum volume that the local weight limit will never allow you to use, and paying tare weight for it on every journey. Choosing insufficient water tank capacity and losing service time to hardened deposits. And comparing a mixer trailer to a truck mixer on purchase price alone, when the real comparison is cost per cubic metre delivered across the asset's working life.
Frequently asked questions
How much does a cubic metre of concrete weigh?
Around 2.4 tonnes for standard structural concrete, with a normal range of roughly 2.3 to 2.5 tonnes depending on the mix and aggregate. Lightweight and heavyweight mixes fall outside this range and should be calculated separately.
How much concrete can a mixer trailer carry legally?
Subtract the tractor unit weight and the trailer tare weight from the national gross combination limit, then divide the remaining payload by 2.4. Under a gross limit of 40 tonnes this may, for example, produce 9 to 10 m³, depending on actual operating mass and axle limits. Under higher national limits the figure rises proportionally.
Is a mixer trailer better than a truck mixer?
For volume per trip and tractor utilisation, generally yes. For manoeuvrability on tight urban sites, no. Most batching plants running significant volume operate both, using trailers for bulk delivery and truck mixers for constrained access.
Does the drum have to keep turning during transport?
Yes. Concrete begins to set from the moment it is batched, and continuous rotation keeps the mix homogeneous and workable. Drive comes either from the tractor's PTO or from an auxiliary engine mounted on the trailer, and that choice determines whether the drum can keep turning while the trailer is uncoupled.
What drum volume should I order?
The one that matches your legal payload, not the largest available. Calculate deliverable volume from your national weight limit first, then choose the drum whose rated mixing capacity sits closest to that figure. STU produces 12.7 m³ and 14.7 m³ drums as standard; 9, 10 and 12 m³ units are built to order and subject to a minimum quantity.
How STU builds mixer trailers
STU Trailers produces concrete mixer trailers for batching plants and producers of ready mixed concrete across Europe, the Middle East and Africa. Standard production volumes are 12.7 m³ and 14.7 m³. Drums of 9, 10 and 12 m³ are built to order and subject to a minimum quantity, so operators working under tighter weight limits should raise volume requirements at the enquiry stage rather than assuming a stock size.
Drum drive is available from the tractor's PTO or an independent auxiliary engine. The water tank holds 600 litres; the admixture tank of 40 litres is optional on the 12.7 m³ model and standard on the 14.7 m³ model. Chassis are cut by laser and welded from S355J2+N structural steel, with S690QL or Strenx 700MC grades with high strength where the design and load case require them, which keeps tare weight down and converts directly into deliverable concrete within a given weight limit. Axles are available from SAF, BPW, GIGANT, VALX, TRAX and MPD in capacities of 9, 11, 12 and 14 tonnes, with WABCO or Knorr-Bremse EBS braking. Drum volume, drive arrangement, tank capacity, chute configuration and axle count are specified per order against the customer's batching cycle, delivery profile and national weight limits.