20mm nominal aggregate is the standard maximum size for general-purpose concretes such as slabs, foundations and footings, because it balances strength, workability and economy better than finer or coarser alternatives. That single fact settles most sizing questions before you even open a mix design guide.
A few clarifications matter before you order a tonne of the stuff:
- 10mm or 14mm aggregate suits thin sections, tight reinforcement, or where a smoother finish is needed (screeds, some decorative work).
- 20mm is the default for slabs 150mm or thicker, footings, foundations and most driveway bases.
- Formal specification should always reference BS EN 12620 and BS 8500, or their ASTM/ACI equivalents if you’re working to US standards. The sections below cover grading, mix ratios and site handling in detail.
Key Takeaways
Choosing 20mm aggregate for general-purpose concrete works because it minimises paste demand while remaining compatible with standard reinforcement spacing and pump lines.
| Point | Details |
|---|---|
| Default to 20mm for general work | Slabs, footings and foundations with normal reinforcement spacing suit 20mm nominal aggregate by default. |
| Drop to 10mm/14mm when congested | Thin sections, tight reinforcement or narrow pump lines need finer aggregate to avoid blockage. |
| Check grading against BS EN 12620 | Confirm delivered material meets BS EN 12620 and your concrete meets BS 8500 before a large pour. |
| Account for surface moisture | Aggregate typically carries around 3% surface moisture, so weigh it before setting batch water. |
| Order from a quarry-direct supplier | Poweraggregates supplies 20mm aggregate from its own Cork quarries, with advice on bag versus bulk quantities. |
Table of Contents
- What does 20mm nominal aggregate size actually mean?
- Why is 20mm aggregate the go-to choice for most concrete?
- What are the most common uses for 20mm aggregate concrete?
- How do you mix 20mm aggregate concrete and manage it on site?
- How do you order and handle 20mm aggregate on delivery?
- What support does Power Aggregates offer for 20mm aggregate projects?
- What matters most when choosing 20mm aggregate concrete?
- Where to get 20mm aggregate for your next concrete pour
- Sources
- FAQ
What does 20mm nominal aggregate size actually mean?
“20mm nominal maximum size” doesn’t mean every stone is exactly 20mm. It means the grading is controlled so that 20mm sieve retains a small percentage of the material, with the bulk passing through and being retained on smaller sieves down the scale. A sieve analysis plots percentage passing against sieve size, and a well-graded 20mm aggregate shows a smooth curve rather than gaps or spikes at any one fraction.
Grading matters more than most people assume. Aggregate typically makes up 60 to 75% of the total volume of concrete, so its shape, size distribution and cleanliness drive workability, strength and durability almost as much as the cement paste itself. Poorly graded material, with too many fines or an uneven distribution across sieve sizes, forces up paste demand: you need more cement and water to fill the gaps and keep the mix workable, which raises cost and increases the risk of shrinkage cracking.
If you’re specifying or buying at any scale, check these before committing:
- BS EN 12620 for aggregate quality and grading limits.
- BS 8500 for the concrete specification itself, including exposure class and strength class.
- ASTM C33 or ACI 211.1 if working to US-influenced standards.
Statistic callout: Poorly graded aggregate can push paste demand up significantly, since gaps in the grading curve have to be filled with cement and water rather than stone. That’s the key reason experts flag grading checks before a large pour, not just a formality for the paperwork.
Why is 20mm aggregate the go-to choice for most concrete?
Bigger aggregate means less surface area to coat with cement paste for the same volume of stone. That’s the entire economic case for 20mm over 10mm: less paste needed, lower cement content for a given strength class, and often a cheaper cube for cube of concrete.
The trade-offs run the other way for placement. Larger aggregate can struggle to pass between closely spaced reinforcement bars, and tight reinforcement congestion or thin members generally call for a smaller maximum size rather than 20mm. Pump lines under about 100mm internal diameter can also choke on 20mm stone, particularly if the mix is under sanded.
Where 20mm earns its place:
- Slabs, footings and foundations of typical residential and light commercial scale.
- Mass pours where economy and bulk strength matter more than fine surface finish.
- Situations with generous reinforcement spacing and straightforward geometry.
Where it’s a poor fit: thin toppings, heavily congested columns or beams, and narrow-bore pumped applications.
Pro Tip: If you’re unsure whether your reinforcement spacing allows 20mm to pass freely, measure the smallest gap on site and compare it against three times the aggregate’s nominal size. That rule of thumb catches most blockage problems before they happen.
What are the most common uses for 20mm aggregate concrete?
Most general building work defaults to 20mm without a second thought, and for good reason. Here’s where it earns that default status, and where it doesn’t:
- Foundations and footings, where bulk strength and economy outweigh surface finish concerns.
- Ground floor slabs, typically 100 to 150mm thick or more, where 20mm passes easily through standard mesh reinforcement.
- Driveways and hardstanding, both the sub base (often using drainage stone products) and the structural slab above it.
- Steps, posts and general groundwork, a use case Heidelberg’s own 20mm gravel guidance specifically recommends.
- Kerbs and edging courses, where 20mm concrete blocks are a common alternative to poured concrete; see typical concrete block applications for comparison.
Switch to 10mm or 14mm for thin screeds, architectural exposed-aggregate finishes, or anywhere pump line diameter is restricted. Recycled aggregates can substitute for natural 20mm stone in some non-structural applications, though grading consistency tends to vary more between batches.
How do you mix 20mm aggregate concrete and manage it on site?
Mix ratios shift with the strength class you need, and 20mm stone underpins most of the standard domestic and light commercial recipes. Illustrative ratios by volume, using 20mm aggregate, run roughly 1:2:4 for C20, 1:2:3 for C25, and 1:1.5:3 for C30, corresponding to about 12, 13 and 15 bags of cement per cubic metre respectively. Treat these as starting points, not gospel: always confirm against BS 8500 or your structural engineer’s specification, since exposure class and reinforcement design can shift the numbers.
Moisture is the variable most site teams under manage. Aggregate delivered in bulk commonly carries around 3% surface moisture, and if you batch water as though the stone is dry, you’ll oversoak the mix, weaken the final cured strength and increase bleeding at the surface. Weigh or estimate moisture content before adding mix water, particularly after rain or overnight storage without cover.
A few site habits prevent most of the common defects:
- Run a trial mix before a large pour. Small shifts in fines content can demand disproportionate changes in water or cement.
- Watch for segregation during pumping or barrowing. Overly wet or badly graded mixes let the coarse fraction separate from the paste.
- Consolidate properly with a poker vibrator on anything thicker than a light screed. Trapped air pockets around 20mm stone are a common cause of surface blowholes.
- Cure promptly and keep the surface damp for at least a few days to control shrinkage cracking.
Statistic callout: Aggregate grading isn’t a one-off decision made at the quarry. Verify what actually arrives on site against your specification and adjust proportions where needed, rather than assuming every load matches the last.
For quick estimating: a standard domestic slab pour of 5 cubic metres at C25 needs roughly 65 bags of cement if hand-batching, plus loose ballast or separate 20mm stone and sand to the ratios above. Add a waste allowance before you place the order.
How do you order and handle 20mm aggregate on delivery?
Getting the volume right avoids both a wasted second delivery and a yard full of leftover stone. Work through it in order:
- Calculate the volume in cubic metres (length x width x depth), then convert to bags or bulk tonnage using your chosen mix ratio.
- Add a waste allowance, typically 5 to 10% for hand pours and closer to 10% for smaller DIY jobs where spillage and over ordering margins are harder to control.
- Choose your delivery format. Small bags suit patch repairs and minor top ups. Bulk bags (commonly around 800kg, often called dumpy bags) suit medium domestic jobs. Loose bulk delivery is most economical for anything over a few cubic metres.
- Inspect on arrival. Check for consistent grading, absence of clay or organic contamination, and note visible moisture, since a soaked load changes your batching water calculation immediately.
- Store under cover where possible, on a hard standing rather than bare ground, to limit further moisture pickup and contamination before use.
Pro Tip: Order bulk bags rather than loose tipped aggregate if site access is tight or storage time will run past a day or two. Bagged material stays cleaner and is far easier to protect from rain than a loose heap.
What support does Power Aggregates offer for 20mm aggregate projects?
Poweraggregates supplies decorative and structural aggregates from its own quarries in Cork, with decades of experience getting the right grading to the right site rather than just moving stone from A to B. That quarry-to-customer model means less guesswork about provenance or consistency between loads.
Practical help available includes:
- Advice on project sizing, helping you convert a slab or footing dimension into an actual order quantity.
- Guidance on bag versus bulk delivery based on access, storage time and job scale.
- Support choosing between 20mm stone, drainage stone and building products depending on whether the application is structural, drainage, or decorative.
For readers ready to order, the shop and concrete blocks pages list current product options and delivery details.
What matters most when choosing 20mm aggregate concrete?
The industry treats 20mm as a default because it usually is the right default, not because anyone’s done the sums for your specific pour. That’s the gap most guidance skips. A slab foundation with generous reinforcement spacing genuinely doesn’t need anyone agonising over aggregate size, 20mm is correct, move on. But the moment reinforcement gets congested, or you’re pumping through a narrow line, the “just use 20mm” advice becomes actively wrong, and too many site teams discover that after the pump has already choked.
What gets underrated is moisture management. Everyone talks about mix ratios and strength classes, and far fewer people actually weigh their aggregate before batching water. A 3% moisture swing sounds trivial until you’ve watched a slab come out chalky and underperform its strength class because the batching assumed dry stone that wasn’t.
My honest read: spend less time debating C20 versus C25 ratios, which most suppliers will get right anyway, and more time checking what actually turns up on the lorry. Grading consistency and moisture content, verified on site rather than assumed from the delivery note, prevent more defects than any amount of ratio tweaking.
Where to get 20mm aggregate for your next concrete pour
Poweraggregates supplies 20mm aggregate direct from its own quarries in Cork, which means the grading consistency you need for a reliable mix doesn’t depend on a middleman blending stone from multiple sources. That matters most on larger pours, where a single inconsistent load can throw out your paste demand calculations halfway through the job.
Whether you’re ordering a few bulk bags for a garden footing or planning bulk delivery for a driveway slab, the team can talk through bag versus bulk formats, waste allowances and delivery scheduling before you commit to a quantity. That’s useful if you’ve never ordered structural aggregate before and aren’t sure how the numbers translate into lorry loads. Browse the decorative stone and aggregates range to see current 20mm stock, or head straight to the shop to check pricing and place an order for your next pour.
Sources
- Aggregates for Concrete (ACI bulletin)
- Aggregates main (Penn State engineering resource)
- Concrete Calculator UK | Cement & Ballast Bags Calculator | TradeCalcs
- Gravel 20mm (Heidelberg Materials user guide)
FAQ
Why use 20mm aggregate in concrete?
20mm aggregate reduces paste demand compared with smaller sizes, because larger stones have less surface area to coat with cement paste for the same volume, lowering both cost and cement content for a given strength class.
Is 20mm gravel OK for concrete?
Yes, 20mm gravel conforming to BS EN 12620 is standard for footings, foundations and general slabs, provided grading and moisture are checked on delivery.
Is 10mm or 20mm aggregate better for concrete?
Neither is universally better. 20mm suits general-purpose pours with normal reinforcement spacing, while 10mm suits thin sections, congested reinforcement, or narrow pump lines where 20mm stone risks blocking.
What does a 20mm nominal size aggregate mean?
It means the grading is controlled so the bulk of the material passes a 20mm sieve with only a small percentage retained, rather than every particle measuring exactly 20mm.
How do I calculate how much 20mm aggregate I need?
Calculate the pour volume in cubic metres, apply your mix ratio (for example 1:2:4 for C20), then add a 5 to 10% waste allowance before ordering bags or bulk delivery.