TOOL 03 — SANDING

Sanding Disc Estimator

Estimate grits and disc count before you buy.

WORKED EXAMPLE

Grit ladder — 150 mm discs

1206 discs0.22 m² per disc
1503 discs0.43 m² per disc
1803 discs0.43 m² per disc

15 discs12 for the job, plus 3 spare

Disc estimate
Value	Measurement
Area to sand	1.12 m²
Coverage, first grit	0.22 m² per disc
Coverage, later grits	0.43 m² per disc
Discs for the job	12
Suggested spares (20 %)	3
Suggested purchase	15

Coverage figures are buying estimates, not performance claims — abrasive quality, pressure and how much stock you are removing all move them.
Later grits are assumed to cover twice the area of the first, because they are cutting less material.
Every grit is rounded up to a whole disc, and never below one disc.

This is the tool's worked example, not your figures: one machined hardwood panel 1400 × 800 mm, one face, clear finish, 150 mm discs. Load it into the form with the example button, or enter your own and press the calculate button to replace it.

This estimator needs JavaScript to calculate in your browser — nothing is ever sent anywhere. Without it, the method works on paper: work out the area, take the start grit for your material and condition and the target grit for your finish from the tables in “How this works”, then divide the area by the coverage per disc at each grit and round up. The panel above shows the full worked example: one machined hardwood panel of 1400 × 800 mm, one face, clear finish, 150 mm discs, giving1.12 m², grits 120–150–180 and a suggested purchase of15 discs.

How this works

Inputs

The area — either identical panel sizes (10–6100 mm a side, 1–20 panels, one or both faces) or a measured total (0.01–100 m²) — plus the material, the surface condition, the finish you are preparing for and the disc size. The advanced field overrides the assumed coverage per disc when you know how your abrasive actually performs.

Method

In panel mode the area is:

area = length × width × panels × faces ÷ 10⁶square metres, unrounded in computation

The grit sequence is a slice of one common ladder — a workshop rule of thumb, not a standard:

ladder = 40 · 60 · 80 · 120 · 150 · 180 · 240 · 320workshop rule of thumb — from your start grit to your target, inclusive

Start grit by condition and material
Condition	Softwood	Hardwood	Sheet (ply/MDF)
Rough sawn	60	40	80
Machined or planed	120	120	120
Previously finished	80	80	80

A previously finished surface starting at 80 assumes a sound film — thick old paint may need a coarser start, or a stripping step before sanding at all.

Target grit by finish
Finish	Target grit
Preparing for paint	150
Clear finish	180
Fine finish	240

Coverage per disc is a buying estimate (workshop rule of thumb, overridable above), scaled by disc area:

Base coverage per disc at 125 mm
Material	Coverage
Softwood	0.25 m²
Hardwood	0.15 m²
Sheet (ply/MDF)	0.30 m²

coverage first = base × (disc ÷ 125)² · coverage later = 2 × coverage first150 mm discs carry 1.44 × the base coverage; later grits cut less material

discs per grit = max(1, ⌈area ÷ coverage⌉) · spares = ⌈0.2 × total⌉

Rounding

The area is unrounded in computation and displayed to two decimals (halves rounding away from zero). Every grit's disc count rounds up to a whole disc and never falls below one; the spares figure rounds up from 20 % of the total. Nothing else rounds.

Limitations

Coverage varies widely with abrasive quality, pressure and how much stock you are removing — the defaults are buying estimates, not performance claims.
The ladder is one common progression, not the only correct one.
This estimator never outputs airflow, extractor capacity or duct figures — see the extraction note in the dust-control panel below.
This is a planning aid, not an engineering or safety certification.

Worked example

SD-1: one machined hardwood panel, 1400 × 800 mm, one face, clear finish, 150 mm discs. Area = 1.4 × 0.8 = 1.12 m². Machined stock starts at 120 grit and a clear finish targets 180, so the sequence is 120, 150, 180. Coverage at the first grit = 0.15 × (150 ÷ 125)² = 0.216 m² per disc; later grits 0.432 m². Discs: ⌈1.12 ÷ 0.216⌉ = 6, then ⌈1.12 ÷ 0.432⌉ = 3 and 3. Total 12; spares ⌈0.2 × 12⌉ = 3; suggested purchase 15 discs. This is the reading shown in the result panel until you calculate your own.

Sources

The dust-control panel below is sourced from the HSE's wood dust guidance¹. The grit ladder, start and target grits and coverage figures are labelled a workshop rule of thumb and carry no source. The full reference is in the ledger below and on the sources page.

Safety — HSE-sourced

Dust control

Wood dust can cause serious health problems, including asthma — carpenters and joiners are four times more likely to get it than other UK workers.¹
Hardwood dust can cause cancer, particularly of the nose.¹
The workplace exposure limits are 3 mg/m³ (as an 8-hour time-weighted average) for hardwood dust and 5 mg/m³ for softwood dust; for mixed dusts, the 3 mg/m³ limit applies to all wood dust present.¹
Sanding is a very dusty job that may need additional protection — a suitable face mask as well as extraction.¹
Never sweep up or use compressed air — clean up with an industrial vacuum cleaner of at least dust Class M.¹
Extraction (LEV) at woodworking machines must be examined by a competent person at least every 14 months.¹

Extraction sizing: this estimator does not size dust extraction and never will from generic assumptions. HSE guidance is that an extraction system must be designed around the number, type and layout of the machines connected to it, using the machine manufacturer's information — or an experienced body's — for the airflow each connection needs. Get those figures from the manufacturer or a competent LEV person.

References

Sources

Ref	Reference	Used by
1	Wood dust — Health and Safety Executive (HSE). https://www.hse.gov.uk/woodworking/wooddust.htm Accessed 2026-08-05.	Sanding Disc Estimator (dust-control panel); orbital sander guide