Introduction
Behind every paint estimate is a chain of simple multiplications — but each step hides assumptions about coverage, coats, and component area that can quietly double or halve your result. This guide derives each formula the Cabinet Paint Calculator uses, so you can verify the math, adjust the assumptions, and understand exactly where your gallons come from.
Formula 1: Paintable Area from Components
Area = Σ(Doors × Adoor × 2) + Σ(Drawers × Adrawer) + Aframes + Apanels
Each term:
- Doors × Adoor × 2 — door count times average door face area, doubled for front and back. A standard shaker door (0.4 m × 0.6 m / 16 in × 24 in) has a face area of 0.24 m² (2.67 ft²); with edges, round to ~0.3 m² (3.2 ft²) per face.
- Drawers × Adrawer — drawer fronts count once (visible face + edges); the box interior is excluded.
- Aframes — face-frame stiles and rails, measured or estimated at 0.5–0.7 m² per linear meter of cabinetry.
- Apanels — exposed end panels, islands sides, and decorative panels, measured individually.
Derivation of the doubling: a door is a thin rectangular solid. Paint covers both large faces (2 × L × W) plus four edges (2 × (L + W) × thickness). For a 0.4 × 0.6 m door at 19 mm thick: faces = 2 × 0.24 = 0.48 m²; edges = 2 × (1.0) × 0.019 = 0.038 m². Total ≈ 0.52 m² (5.6 ft²). The calculator’s default per-door value already includes edges.
Formula 2: Enamel Quantity
Gallonsenamel = ceil(Area ÷ Coverageenamel × Coats)
Where Coverageenamel ≈ 9–14 m²/L (100–150 ft²/gal) per coat. The ceiling function rounds up to whole gallons because paint is sold in whole units — a 1.3-gallon requirement means buying 2 gallons.
Why enamel coverage is lower than wall paint: wall paint is formulated for porous drywall and applied thick with a roller (~350–400 ft²/gal). Cabinet enamel goes on thinner (to avoid drips on vertical surfaces) over non-porous primed wood, and the fine finish demands more careful — thinner — application.
Formula 3: Primer Quantity
Gallonsprimer = ceil(Area ÷ Coverageprimer × Primer coats)
Coverageprimer ≈ 18–28 m²/L (200–300 ft²/gal). One coat is standard; use two when going from dark stain to white paint, or over bare wood with heavy tannin bleed (oak, mahogany).
Worked Derivation: The 24-Door Kitchen
Inputs: 24 doors, 12 drawer fronts, 6 linear meters of frames.
Step 1 — Door area. 24 × 0.52 m² (both faces + edges, derived above) = 12.48 m².
Step 2 — Drawer area. 12 × 0.25 m² = 3.0 m².
Step 3 — Frame area. 6 m × 0.6 m²/m = 3.6 m².
Step 4 — Total. 12.48 + 3.0 + 3.6 = 19.08 m² (≈ 205 ft²).
Step 5 — Enamel. 19.08 ÷ 12 m²/L = 1.59 L/coat × 2 coats = 3.18 L → 1 gallon (3.785 L). Tight — many painters would buy 2 gallons for color-matching safety on future touch-ups.
Step 6 — Primer. 19.08 ÷ 23 = 0.83 L → 1 gallon.
Note how the per-door derivation (0.52 m²) gives a smaller total than the quick-estimate default (0.4 m² × 2 = 0.8 m² per door → 19.2 m² doors alone). The difference is the calculator’s conservative default, which builds in margin for oversized doors and waste. When in doubt, the conservative number is the safer buy — leftover enamel stores well for touch-ups. Measurement technique is covered in the how-to-estimate guide.
Unit Conversions Used
| Conversion | Factor |
|---|---|
| m² → ft² | × 10.764 |
| Liters → gallons | ÷ 3.785 |
| m²/L → ft²/gal | × 40.75 |
Worked Example 2: Small Vanity / Single Bathroom Cabinet
Inputs: 4 doors, 2 drawer fronts, 1.2 linear meters of frames.
Metric: 4 × 0.52 m² = 2.08 m² doors; 2 × 0.25 m² = 0.5 m² drawers; 1.2 × 0.6 = 0.72 m² frames. Total = 3.3 m².
Imperial: 4 × 5.6 ft² = 22.4 ft² doors; 2 × 2.7 ft² = 5.4 ft² drawers; 4 ft × 6.5 ft²/ft = 26 ft² frames. Total ≈ 54 ft² (3.3 m² × 10.764 ≈ 35.5 ft² — note the per-component rounding inflates the imperial estimate slightly; always derive from your own measurements).
Enamel at 12 m²/L (130 ft²/gal): 3.3 ÷ 12 = 0.28 L/coat × 2 = 0.55 L → buy 1 quart / 1 liter (the smallest practical unit; quarts cost disproportionately more per liter, but a full gallon is wasteful here). Primer: 3.3 ÷ 23 = 0.14 L → 1 quart.
Worked Example 3: Large Kitchen with Island and Pantry
Inputs: 36 doors, 16 drawer fronts, 9 linear meters of frames, an island with 4 exposed sides (6 m² / 65 ft² of panels), and a tall pantry unit (2 doors counted above, plus 1.5 m² / 16 ft² of exposed side panel).
Metric: 36 × 0.52 = 18.72 m² doors; 16 × 0.25 = 4.0 m² drawers; 9 × 0.6 = 5.4 m² frames; island 6.0 m²; pantry panel 1.5 m². Total = 35.62 m².
Imperial: 36 × 5.6 = 201.6 ft² doors; 16 × 2.7 = 43.2 ft² drawers; 29.5 ft × 6.5 = 191.75 ft² frames; island 65 ft²; pantry panel 16 ft². Total ≈ 517 ft².
Enamel at 12 m²/L: 35.62 ÷ 12 = 2.97 L/coat × 2 coats = 5.94 L ÷ 3.785 = 1.57 gal → buy 2 gallons, and most painters would take a third for color-match safety. Primer at 23 m²/L: 35.62 ÷ 23 = 1.55 L → 1 gallon (a second if two primer coats are needed over dark stain).
Imperial/Metric Side-by-Side Derivation
The calculator accepts both unit systems, but the underlying math is identical — only the constants change. Here’s the full chain in both systems for the 24-door kitchen from Example 1 (19.08 m² / 205 ft²):
| Step | Metric | Imperial |
|---|---|---|
| Door face area | 0.4 m × 0.6 m = 0.24 m² | 16 in × 24 in = 384 in² = 2.67 ft² |
| Both faces | 0.48 m² | 5.33 ft² |
| + edges (19 mm / 0.75 in thick) | + 0.038 m² ≈ 0.52 m² | + 0.25 ft² ≈ 5.6 ft² |
| Total area (24 doors + 12 drawers + 6 m frames) | 19.08 m² | 205 ft² |
| Coverage rate (enamel) | 12 m²/L | 130 ft²/gal |
| Liters/gallons per coat | 19.08 ÷ 12 = 1.59 L | 205 ÷ 130 = 1.58 gal |
| Two coats | 3.18 L | 3.15 gal |
| Rounded purchase | 1 gal (3.785 L) — tight | 1 gal — tight; buy 2 for safety |
The two systems agree (3.18 L ÷ 3.785 = 0.84 gal per the metric path — the small discrepancy comes from rounding the coverage rate). The takeaway: pick one system and stay in it. Converting mid-calculation and rounding at each step compounds error.
Edge-Case Adjustments
Islands
Islands break the “doors × 2 faces” model because their sides are large flat panels, not doors. Measure each exposed side as a rectangle (height × width) and add it to Apanels. A typical 2.4 × 0.9 m (8 × 3 ft) island with finished sides on three faces adds roughly 5–6 m² (54–65 ft²). Decorative corbels or applied moldings add another 10–15%.
Pantries and tall units
Tall pantry cabinets have outsized doors (often 0.5 × 1.8 m / 20 × 72 in — about 1 m² or 10 ft² per face) and large exposed side panels. Don’t fold these into the “average door size” — measure them individually.
Glass-front doors
For glass-front doors, the paintable area is the frame (stiles and rails) only — but the visible interior (shelves, back panel, interior sides) must be added. Net effect: a glass-front cabinet often needs more paint than a solid door, not less.
Open shelving replacing uppers
Open shelves have roughly double the area of closed cabinets per linear meter — every shelf is painted top and bottom plus edges. A 3 m (10 ft) run of three shelves is about 5.4 m² (58 ft²) versus ~3 m² for equivalent closed uppers.
Repaint vs. first-time bare wood
Bare wood absorbs 20–40% more primer than previously finished wood — multiply the primer quantity by 1.3 for bare wood. The enamel quantity is unaffected.
Related Guides
- Complete Guide — full estimating walkthrough with examples.
- Cost Guide — turning quantities into budgets.
- Pro Tips — application advice from the trade.
- FAQ — quick formula answers.
Get Your Number
The formulas are simple; the accuracy lives in the inputs. Count carefully, use enamel (not wall-paint) coverage rates, and round up. The Cabinet Paint Calculator runs all six steps instantly — this guide just shows its work.
Disclaimer: Formulas use typical coverage rates; actual coverage varies by product, surface, and application method. Check manufacturer technical data sheets for exact figures.
