About GardenCalc.uk | Free Garden Calculators for UK Gardeners

What is GardenCalc?

GardenCalc is a collection of 20 free gardening calculators and tools built specifically for UK gardeners. You will not find a signup wall or a paywall here, just straightforward tools that help you work out exactly what you need for your garden project, from a bag of grass seed to a lorry-load of ready-mix concrete.

Every calculator uses UK-standard measurements (metres, litres, kilograms), references UK product sizes and bag weights, and includes current 2026 pricing so you can budget accurately. Whether you are seeding a new lawn, filling a raised bed, building a patio or planning what to sow this month, our calculators give you a clear, accurate answer in seconds.

The site launched in early 2026 and has grown into a library of gardening calculators and guides, including an interactive planting calendar covering 30+ UK vegetables.

Meet Gary — The Man Behind the Numbers

Most of what I know about gardening I learned from my nan and grandad. Their garden was the proper kind: chickens out the back, a pear tree, two yellow plum trees, rhubarb that came up every spring whether you wanted it or not, cabbages, potatoes, the lot. A proper secret garden, the sort that fed the whole family.

Grandad built a bridge across the pond. He was the kind of man who built bridges. Over the years they stocked the pond with koi carp, which got more expensive year on year as they grew. He never stopped adding and improving. That was the standard.

My nan worked alongside him. She really loved it she did. The garden was theirs together and you could feel it the second you walked through the gate.

That is where I learned the only rule that matters: do the maths before you buy. Grandad never bought a single bag of compost more than he needed. He would pace the bed, scratch a number on the back of a packet, and that was that. No second trip. No leftovers going hard at the back of the shed.

I'm not a gardener. I'm ex-Army, and I build the numbers. GardenCalc started with keyword research. People across the UK were searching how much soil, gravel or seed to buy, and the answers they got back were guesses. So I took Grandad's habit and turned it into calculators, built from published UK data: the RHS, the Met Office, British Standards and the suppliers' own figures.

The gardening side I'm still learning, from real gardeners. I volunteer at community gardens and I ask the people who've done it for years. When a page gives you advice, it comes from a named source, not from me pretending I've done the job.

If you spot something wrong on the site, hit reply on any newsletter or email hello@gardencalc.uk. I read every one.

— Gary

How Our Calculators Work

Every calculator on GardenCalc follows the same principles:

Transparent formulas — Each calculator shows you how the maths works, not just the answer. Length × width × depth gives volume; volume × density gives weight; weight divided by bag size gives the number of bags. No black boxes.
UK standards — All measurements are in metric (metres, litres, kilograms) with imperial options where commonly used. Product sizes match what you will find in UK DIY stores: 25kg bags, 40-litre bags and bulk bags. Bulk bags vary by product and supplier, from about 500 litres to 1 cubic metre (1,000 litres), so check the listing before you order.
Real wastage factors — Where cuts and offcuts eat into your materials, as with paving or turf, the calculator adds a wastage margin and the page tells you how much. A paving calculator that does not account for cuts and breakages is not useful.
Current pricing — Where we include cost estimates, they reflect actual UK retail prices in 2026, updated regularly. We check prices at major UK retailers (B&Q, Wickes, Screwfix, Amazon) to give you realistic budget figures.
No registration required — Every calculator works instantly, for free, without creating an account or entering personal information. We believe useful tools should be accessible to everyone.

Our 20 Calculators

Grass Seed Calculator — work out how much grass seed you need for new lawns, overseeding or patching bare spots
Lawn Feed Calculator — calculate lawn feed quantities for any product type and lawn size
Fertiliser Calculator — get the right amount of fertiliser per square metre for lawns, borders or veg patches
Mulch Calculator — find out how much bark, wood chip or decorative mulch you need for borders and beds
Topsoil Calculator — work out how much topsoil you need in litres, bags or bulk bags for beds, borders and lawns
Compost Calculator — calculate compost quantities in litres, bags and bulk bags for any project
Water Butt Calculator — find out how much rainwater your roof can collect and what size water butt you need
Gravel Calculator — calculate gravel, pea shingle and decorative stone for driveways, paths and patios
Paving Calculator — work out how many slabs you need for patios, paths and garden areas
Concrete Calculator — work out concrete for slabs, shed bases, fence posts and footings in bags or ready-mix
Decking Calculator — calculate boards, joists, screws and costs for any deck size
Sub-Base Calculator — calculate MOT Type 1 hardcore quantities in tonnes and bulk bags
Tarmac Calculator — calculate tarmac or asphalt quantities for driveways, paths and car parks in tonnes and cost
Bark Chippings Calculator — work out bark for paths, play areas and borders with BS EN 1177 play bark depths
Frost Date Calculator — find your last frost and first frost dates by UK postcode or region
Plant Spacing Calculator — work out how many plants fit your bed in grid, triangular or row patterns
Raised Bed Calculator — how much soil for a raised bed, with 60/30/10 mix breakdown for rectangular and L-shaped beds
Turf Calculator — work out how many rolls of turf you need for any lawn shape, with wastage allowance and cost estimate
Decorative Stone Calculator — calculate slate, Cotswold and limestone chippings in tonnes, bulk bags and 25kg bags
Lawn Top Dressing Calculator — work out how much sand-soil top dressing you need in kilograms, bags and bulk bags

Planting Guides

Beyond calculators, we publish an in-depth interactive planting calendar covering 30+ UK vegetables with sowing, transplanting and harvest dates, so you can see exactly what to plant and when.

Editorial Standards

All content on GardenCalc is written and edited by Gary Hodson, working from the published UK sources listed below. We follow these editorial principles:

Accuracy first — Every calculation formula is verified against industry standards (RHS guidelines, BS standards, manufacturer specifications). If we are not confident in a number, we do not publish it.
Sourced, not made up — I'm not a gardener and I don't pretend to be. The practical advice on this site comes from published UK sources and from real gardeners I learn from, not from jobs I haven't done.
Regular updates — Product prices, availability and specifications change. We review and update our content regularly to keep it accurate. If you spot something outdated, please let us know.
Honest recommendations — Some pages include affiliate links to products on Amazon and other retailers. We earn a small commission if you buy through these links, at no extra cost to you. Some links point to related kit for the project, not only the materials the calculator works out. Affiliate relationships never influence our calculator results or editorial advice.

Data sources

The numerical claims and calculation formulas on this site are cross-referenced against the following published authorities. Where a specific rate or density is used in a calculator or in page content, it is drawn from one of these sources:

RHS (Royal Horticultural Society) — lawn feeding rates, fertiliser application guidance, sowing and planting calendars, pest & disease advice. rhs.org.uk/advice
Met Office — 30-year UK climate averages (1991 to 2020) for regional rainfall. Our frost dates are worked out by GardenCalc from the Met Office HadUK-Grid gridded observations for 1991 to 2020. The Met Office does not publish frost dates by postcode. metoffice.gov.uk climate averages
BS 3882 (British Standard for topsoil) — topsoil specification classes used in the soil and topsoil calculators.
EN 1177 (European safety standard for impact-absorbing surfaces) — play bark minimum safety depths used in the play bark calculator.
Seed and fertiliser manufacturer packaging — application rates cross-checked against current UK retail product labels (Miracle-Gro, Westland, Vitax, Johnsons, DLF and others). Rates on bags vary slightly by product; we use the midpoint of common UK retail rates.
Local UK garden-centre pricing (2026) — spot-checked against B&Q, Wickes, Homebase and Amazon UK at time of content publication. Prices change; flag anything that looks off at hello@gardencalc.uk.

How our content is produced

Content on GardenCalc is researched, written and published by Gary Hodson, working from published UK data and sources. For longer-form narrative content on calculator pages (the practitioner guidance that sits alongside the calculator itself), draft text is produced collaboratively using AI writing assistants (Claude by Anthropic) working from Gary's notes, outline and specific UK data, then reviewed, fact-checked, voice-edited and signed off by Gary before publication. All calculator formulas, regional data, sourcing decisions, product recommendations, and the site's core editorial judgement are Gary's direct work. Each page shows a "Last reviewed" date indicating when Gary last verified the numbers and content.

We publish a free newsletter, covering seasonal planting tips, calculator updates and practical, UK-specific gardening advice with no fluff. You can subscribe here. We send no spam, we never sell your email, and you can unsubscribe any time.

Get in Touch

Have a suggestion for a new calculator? Spotted something that does not look right? Want to share how a calculator helped with your project? I would love to hear from you.

Email: hello@gardencalc.uk

I also build this thing in public on X, mistakes included.