Grout Quantity Calculator: Tile Size, Joint Width & Coverage

Summary

Accurately calculating grout quantities saves money (no excess bags opened and wasted) and prevents costly mid-job delays (running out of grout and needing an emergency supplier run). The calculation is straightforward once the variables are understood, but many tradespeople either estimate informally and over-buy, or trust the back-of-bag coverage figures which don't account for joint depth, tile format, or absorption.

The key variables are tile size (which determines joint perimeter per m²), joint width, joint depth (which depends on tile thickness and installation method), and grout density. These combine multiplicatively — a small change in joint width has a surprisingly large effect on grout quantity. Wider joints mean much more grout per m² and also mean a different grout type (sanded vs unsanded).

This article provides worked examples for the most common UK tiling scenarios and a calculation method that can be applied to any tile/joint combination.

Key Facts

• Sanded grout: For joint widths 3–15mm; sand particles fill larger voids; slightly higher density than unsanded
• Unsanded grout: For joint widths up to 3mm (mosaics, rectified porcelain, polished marble); no sand particles that could scratch soft surfaces
• Typical cementitious grout density: 1.5–1.8 g/cm³ (1,500–1,800 kg/m³) — varies by product
• Typical epoxy grout density: ~1.6–2.0 g/cm³; heavier but covers similarly per joint volume
• Joint depth for floor tiles: Typically 2/3 of tile thickness — don't fill flush to the face; leave 1–2mm from face for sealing / expansion
• Joint depth for wall tiles: 2/3 of tile thickness as a guide; in practice grout is applied to fill available depth
• Coverage varies significantly by tile porosity: Porous tiles (terracotta, natural stone) absorb water from grout — need more mixing water and potentially more grout
• Wastage factor: Add 10–15% for standard layouts; 15–20% for mosaic, complex patterns, or porous surfaces
• Bag sizes: Cementitious grout typically sold in 2.5kg, 5kg, 10kg, and 25kg bags — round up to nearest bag
• Coverage figures on bags: Usually based on a specific tile size and joint width — read the small print; they are rarely the same scenario as your job

Grout Quantity Formula

Volume of grout joints per m² (cm³):

Grout volume = [(2 × (tile length + tile width)) / (tile length × tile width)] × joint width × joint depth × 10⁶

Simplified for practical use:

Grout (kg/m²) = [(2 × (L + W)) / (L × W)] × Jw × Jd × density

Where:

• L = tile length (mm)
• W = tile width (mm)
• Jw = joint width (mm)
• Jd = joint depth (mm)
• Density = grout density (kg/litre — typically 1.5–1.8)

Worked Examples

Example 1: Standard Floor Tile — 300×300mm

Tile: 300×300mm porcelain, 10mm thick Joint width: 5mm (standard floor joint) Joint depth: 7mm (approx 2/3 of tile thickness) Grout density: 1.7 kg/litre (standard cementitious grout)

Step 1 — Joint perimeter per m²:

• Single tile perimeter = 2 × (300 + 300) = 1,200mm
• Single tile area = 300 × 300 = 90,000mm²
• Joint perimeter per m² = (1,200 / 90,000) × 1,000,000 = 13,333 mm/m²

Step 2 — Joint volume per m²:

• Volume = 13,333 × 5mm × 7mm = 466,667 mm³/m² = 0.467 litres/m²

Step 3 — Grout weight per m²:

• Weight = 0.467 litres × 1.7 kg/litre = 0.79 kg/m²

Step 4 — Add 15% wastage:

• 0.79 × 1.15 = 0.91 kg/m² (approximately 1 kg/m²)

For a 20m² floor: 20 × 0.91 = 18.2 kg → buy four 5kg bags (20kg total) or one 25kg bag.

Example 2: Large Format Tile — 600×600mm

Tile: 600×600mm porcelain, 12mm thick Joint width: 3mm (rectified large format, minimum joint) Joint depth: 8mm Grout density: 1.7 kg/litre

Step 1 — Joint perimeter per m²:

• Tile perimeter = 2 × (600 + 600) = 2,400mm
• Tile area = 600 × 600 = 360,000mm²
• Joint perimeter per m² = (2,400 / 360,000) × 1,000,000 = 6,667 mm/m²

Step 2 — Joint volume:

• Volume = 6,667 × 3 × 8 = 160,008 mm³/m² = 0.160 litres/m²

Step 3 — Grout weight:

• 0.160 × 1.7 = 0.27 kg/m²

Step 4 — Add 15% wastage:

• 0.27 × 1.15 = 0.31 kg/m²

For a 30m² floor: 30 × 0.31 = 9.3 kg → buy two 5kg bags (10kg total).

Note: Large format tiles with narrow joints use dramatically less grout than smaller tiles with wider joints — this surprises many tradespeople.

Example 3: Mosaic Tiles — 50×50mm

Tile: 50×50mm glass mosaic, 4mm thick Joint width: 2mm (typical mosaic joint) Joint depth: 3mm Grout density: 1.6 kg/litre (fine unsanded grout)

Step 1 — Joint perimeter per m²:

• Tile perimeter = 2 × (50 + 50) = 200mm
• Tile area = 50 × 50 = 2,500mm²
• Joint perimeter per m² = (200 / 2,500) × 1,000,000 = 80,000 mm/m²

Step 2 — Joint volume:

• Volume = 80,000 × 2 × 3 = 480,000 mm³/m² = 0.480 litres/m²

Step 3 — Grout weight:

• 0.480 × 1.6 = 0.77 kg/m²

Step 4 — Add 20% wastage (mosaic is harder to grout cleanly):

• 0.77 × 1.20 = 0.92 kg/m²

For a 15m² mosaic floor: 15 × 0.92 = 13.8 kg → buy three 5kg bags (15kg total).

Key insight: Despite narrower joints (2mm vs 5mm), the mosaic produces nearly the same grout quantity as the 300×300mm floor tile because there are so many more joints per m².

Example 4: Wall Tile — 200×100mm Metro/Brick Pattern

Tile: 200×100mm ceramic wall tile, 7mm thick Joint width: 3mm (standard wall tile) Joint depth: 5mm Grout density: 1.65 kg/litre

Step 1 — Joint perimeter per m²:

• Tile perimeter = 2 × (200 + 100) = 600mm
• Tile area = 200 × 100 = 20,000mm²
• Joint perimeter per m² = (600 / 20,000) × 1,000,000 = 30,000 mm/m²

Step 2 — Joint volume:

• Volume = 30,000 × 3 × 5 = 450,000 mm³/m² = 0.450 litres/m²

Step 3 — Grout weight:

• 0.450 × 1.65 = 0.74 kg/m²

Step 4 — Add 15%:

• 0.74 × 1.15 = 0.85 kg/m²

For a 12m² tiled wall: 12 × 0.85 = 10.2 kg → buy two 5kg bags (10kg, plus 0.2kg marginal deficit) → buy three 5kg bags (15kg) to be safe, or one 10kg + one 5kg.

Quick Reference Table

Frequently Asked Questions

Do I need more grout for natural stone tiles?

Yes. Natural stone tiles (limestone, travertine, sandstone) are porous and absorb water from the grout mix during application. This means: (a) more mixing water is needed to maintain workability as the tile absorbs, (b) the grout shrinks more as it dries (excess water lost into the tile), and (c) you may need a second pass to fill joints that have sunk. Add 20–25% wastage rather than 15% for highly porous materials. Always seal the stone before grouting to prevent absorption.

My tile supplier says one 5kg bag covers 10m² — does that match your calculation?

Coverage figures on bags are typically calculated for a specific standard scenario (often 300×300mm at 3mm joint width). If your joint width, tile size, or joint depth differs, the coverage will differ substantially. Always recalculate for your specific tile and joint specification rather than trusting the bag figure.

Can I use the same grout for both floor and wall tiles?

Many cementitious grouts are suitable for both, but check the manufacturer's data sheet. Some wall grouts are formulated for faster drying and lower sag — these may not be suitable for floor applications. Floor tile grout must be suitable for the expected traffic load and, if in a wet area, must be waterproof or sealed after installation. Always check the technical data sheet for the specific product.

What is the difference between grout weight and grout volume for ordering?

Grout is sold by weight (kg), not volume. The formula above calculates volume (litres) and then converts using the density. Different products have different densities — always check the product data sheet for the density of the specific grout you're using. Standard cementitious grouts are approximately 1.5–1.8 kg/litre when dry; mixed grout is heavier. Order by weight.

Regulations & Standards

• BS 5385 Part 1 (Wall and floor tiling) — specification includes grout joint dimensions and application standards

• BS EN 12808 (Grouts for tiles) — grout performance classification (cementitious: CG1/CG2; epoxy: RG)

• Manufacturer data sheets — always the primary reference for mixing ratios, coverage, open time, and cure time for specific products

• Mapei Grout Coverage Calculator — online tool for specific Mapei grout products

• BAL Technical Guidance — grout selection and coverage guidance

• Tile Association Technical Bulletins — joint width recommendations and grout selection

• Ardex Grout Technical Data Sheets — product-specific coverage and mixing data

• tiling tools — tools for grout application and finishing

• mosaic tiling — mosaic-specific grouting techniques

• large format tiles — grout joint requirements for large format tiles

• plaster quantities — similar quantity calculation approach for plastering