Published: Aug 31, 2026 | By: Dovaformwork

How Many Acrow Props Do You Need for a Slab?

formwork insights

One of the most common questions contractors ask when preparing for a suspended slab is:

“How many Acrow props do I need?”

It sounds like a simple calculation.

It isn’t.

You cannot reliably determine the number of Acrow props from floor area alone. The required number depends on the slab, the formwork arrangement, beam spacing, support height, loads and the capacity of the selected props.

So if someone tells you:

“Just use two props per square metre.”

“For a 500m² slab, you need X number of props.”

without asking anything else, be careful.

The better approach is to calculate the temporary support system around the actual project.

What do Acrow props actually support?

An Acrow prop is a vertical adjustable steel support used in temporary works.

In a typical slab formwork arrangement, the props support the primary beams, while secondary beams and plywood or other formwork sheathing sit above them.

A simplified arrangement looks like this:

Concrete slab → Formwork sheathing → Secondary H20 beams → Primary H20 beams → Acrow props

The load therefore moves through the system.

That is why prop quantity cannot be considered separately from the H20 beam arrangement.

H20 timber beams and adjustable props are commonly used together in slab formwork systems.

So how do you estimate the number of props?

The basic process is:

1. Start with the slab and formwork layout

You need to know the slab area, thickness, dimensions and clear height.

You also need to understand how the formwork is being arranged.

For example:

Where will the primary beams run?
How far apart will they be?
What spacing is required between supports?
What secondary beam spacing is being used?
What happens at edges, beams and openings?

2. Determine the primary beam arrangement

Primary beams carry the secondary members and transfer their loads into the supporting props.

The spacing should come from the temporary-works or formwork design.

It should not simply be copied from another project because two slabs can have the same area but very different support requirements.

3. Place the props under the primary beams

The props then support the primary beams at the required intervals.

That gives you a practical way to estimate the quantity.

A simplified example:

Suppose the temporary-works arrangement requires approximately 1.2m between props along a primary beam.

If one primary beam is 12m long:

12 ÷ 1.2 = 10 support spaces

Depending on the actual layout and end conditions, that might mean roughly 11 support positions along that beam.

Now multiply that by the number of primary beam lines.

That gives you a starting quantity.

It is still not a final structural design.

Example: a 300m² slab

Imagine a rectangular slab measuring:

15m × 20m = 300m²

Someone might be tempted to say:

“300m² means 300 props.”

That is not how the system should be calculated.

Instead, the support layout needs to be established first.

For illustration only, suppose the formwork design creates 10 primary beam lines, each approximately 20m long, with support positions arranged at a nominal 1.25m interval.

That would give approximately:

20 ÷ 1.25 = 16 support spaces per beam line

With end support positions included, the arrangement could require roughly 17 props per line.

Across 10 lines:

17 × 10 = 170 props

But notice what happened.

The answer came from the beam and support layout, not simply from dividing the floor area by a magic number.

And this is only an illustrative calculation.

The actual quantity must follow the project’s temporary-works design, prop capacity and loading conditions.

What changes the number of Acrow props you need?

Several things.

Slab thickness

A thicker slab generally means greater dead load.

That can affect the required support arrangement.

Clear height

The working height of the prop matters.

An Acrow prop operating at one extension range does not necessarily have the same capacity as another size or configuration.

Beam spacing

Closer primary beam spacing can change the load carried by each support.

The same applies to secondary beam spacing and the formwork sheathing.

Prop capacity

Different props and extension conditions can have different working capacities.

Do not assume that every “Acrow prop” has the same capacity simply because it has the same general appearance.

Construction loads

The temporary works may need to account for more than the fresh concrete.

Formwork, reinforcement, workers, equipment and construction activity can all matter to the design.

Back-propping

The support requirement below the active slab can also change depending on the structural sequence and back-propping arrangement.

Openings and irregular geometry

A simple rectangular slab is easier to lay out than one with large openings, transfer areas, deep beams or complicated edges.

Why the “props per square metre” rule can be misleading

You may see online examples suggesting a simple number such as:

1 prop per 1.5m²

2–3 props per m².

These numbers may be useful as rough discussions in particular systems or projects, but they should not be treated as a universal design rule.

One published example, for instance, describes approximate prop spacing for a standard slab system alongside specific H20 beam spacing assumptions. Change the slab, beam arrangement, support height or loading and the calculation changes.

So the better question is not:

“How many props per square metre?”

It is:

“What spacing and support arrangement does this slab require?”

That distinction can save you from ordering too little—or unnecessarily paying for equipment you do not need.

What size Acrow prop do you need?

This is another area where contractors sometimes order too quickly.

Acrow props are available in different sizes and extension ranges.

The correct prop depends on the required height and the working capacity at that configuration.

For example, Dova’s Acrow prop range includes different sizes for varying clear heights and temporary-support applications.

But height alone is not enough.

The selected prop also needs to be appropriate for the load it is expected to carry.

The safest process is:

Determine the load → determine the required height → check the appropriate prop capacity → establish the spacing.

What information should you send when requesting a prop quote?

If you want an accurate quantity and quotation, don’t send only:

“I need Acrow props for a slab.”

Send as much of the following as you can:

Slab area

Slab thickness

Clear height

Structural drawings

BOQ

Formwork layout

Construction stage

Project location

Required duration

And tell the supplier whether you want to:

BUY

RENT

the equipment.

That gives the supplier something useful to work with.

Should you buy or rent Acrow props?

That depends on what happens after the current project.

Renting can make sense when:

You’re working on one defined project.

You need temporary support for a specific construction stage.

You don’t want to tie up capital in equipment you’ll rarely use again.

You need additional props to supplement your existing stock.

Buying can make sense when:

You run projects regularly.

You expect repeated use across multiple developments.

You want to build your own temporary-works inventory.

You have storage and logistics arrangements in place.

Don’t compare rental price and purchase price alone.

Think about:

Project duration + expected reuse + transport + storage + maintenance + future demand.

A common mistake: ordering before checking the whole system

A contractor may know they need:

200 Acrow props.

But do they also have enough:

H20 beams?
Forkheads?
Marine boards?
Accessories?
Additional support for openings?
Back-propping equipment?

The prop is only one part of the system.

That is why a good equipment requirement should be looked at as a complete formwork arrangement.

Need help calculating your requirement?

If you’re currently arranging Acrow props for a project, don’t guess your way through the procurement.

You can now easily Send Dova your required quantity, prop size/height, project location and when you need them.

If you’re not sure about the quantity or size, send the BOQ, drawing or basic slab details instead.

We’ll check the available stock and give you the supply/rental option, expected delivery arrangement and the equipment details before you commit.

That’s the part that matters when you’re trying to keep a project moving.

Final takeaway

There is no single number of Acrow props that is correct for every slab.

The quantity depends on the formwork layout, beam arrangement, support spacing, loads, clear height and prop capacity.

So don’t begin with: