Máy Tính Sơn – Ước Tính Phủ Phòng
Ước tính lượng sơn cần thiết dựa trên kích thước phòng và số lớp sơn. Máy tính xây dựng miễn phí, kết quả tức thì.
How to Calculate Paint Coverage
Estimating the right amount of paint saves money and prevents mid-project trips to the store. Standard interior latex paint covers approximately 350–400 square feet per gallon on a single coat over a smooth, primed surface. Rough or porous surfaces such as bare drywall, textured walls, or unpainted masonry may reduce coverage to 250–300 square feet per gallon.
The basic formula for wall paint is:
Gallons needed = (Total wall area − door/window deductions) × number of coats ÷ coverage rate
To find total wall area, calculate the room perimeter (sum of all wall lengths) and multiply by the ceiling height. Then subtract openings: about 20 square feet per standard interior door and 15 square feet per average window.
Worked example: A 12 ft × 14 ft room with 9 ft ceilings, 1 door, and 2 windows. Perimeter = (12 + 14) × 2 = 52 ft. Wall area = 52 × 9 = 468 sq ft. Subtract 1 door (20 sq ft) + 2 windows (30 sq ft) = 50 sq ft. Net paintable area = 418 sq ft. At 400 sq ft per gallon, one coat requires about 1.05 gallons. For two coats: 2.1 gallons — buy 3 gallons to have a safety margin and touch-up reserve.
For ceilings, simply use length × width. A 12 × 14 ft room has a 168 sq ft ceiling, which needs about 0.4–0.5 gallons per coat. One gallon is usually plenty for a single room ceiling with two coats.
Paint Coverage Rates by Surface Type
Coverage varies significantly depending on the surface you are painting. The table below provides realistic coverage rates for common residential surfaces.
| Surface type | Coverage per gallon (sq ft) | Notes |
|---|---|---|
| Smooth drywall (primed) | 350–400 | Best-case scenario; standard for estimating |
| Bare / new drywall (unprimed) | 250–300 | Porous surface absorbs more paint; always prime first |
| Textured walls (orange peel, knockdown) | 250–350 | Texture peaks consume extra paint |
| Stucco / masonry | 150–250 | Very porous; may need block filler before topcoat |
| Wood siding (painted) | 300–400 | Previously painted surfaces cover well |
| Wood siding (bare) | 200–300 | Prime with oil-based or shellac primer |
| Metal (primed) | 350–500 | Non-porous surface stretches paint further |
| Brick / concrete block | 100–200 | Extremely porous; use elastomeric or masonry paint |
These rates assume a single coat applied with a roller. Spraying typically uses 20–30 % more paint due to overspray and atomisation loss. Brushing uses about the same amount as rolling but is slower. When in doubt, use the lower end of the coverage range for budgeting.
Primer, Paint Types, and Finishes
Choosing the right primer and finish is just as important as calculating the amount. Using the wrong combination wastes time, money, and leads to peeling or poor adhesion.
Primer selection guide:
| Primer type | Best for | Dry time | Coverage (sq ft/gal) |
|---|---|---|---|
| Latex / water-based | New drywall, previously painted surfaces, light colour changes | 1–2 hours | 300–400 |
| Oil-based (alkyd) | Bare wood, stain blocking, adhesion on glossy surfaces | 8–24 hours | 300–400 |
| Shellac-based (BIN) | Severe stains (smoke, water, marker), odour blocking | 30–60 min | 300–400 |
| Bonding primer | Glossy surfaces, tile, laminate, metal | 1–2 hours | 250–350 |
| PVA drywall primer | New drywall only — seals paper face | 1 hour | 350–450 |
Paint finish guide:
| Finish | Sheen level | Washability | Best rooms |
|---|---|---|---|
| Flat / matte | 0–5 % | Low — marks easily | Ceilings, low-traffic bedrooms, formal living rooms |
| Eggshell | 10–25 % | Moderate | Living rooms, dining rooms, hallways |
| Satin | 25–35 % | Good — wipes clean | Family rooms, kids rooms, hallways, laundry |
| Semi-gloss | 35–70 % | Excellent — moisture resistant | Kitchens, bathrooms, trim, doors, cabinets |
| High gloss | 70–90 % | Maximum durability | Doors, furniture, accent pieces, exterior shutters |
Higher-sheen paints are more durable and washable but show surface imperfections more readily. If your walls have dents, patches, or uneven texture, flat or eggshell will hide them better. Bathrooms and kitchens benefit from semi-gloss due to moisture and grease exposure.
Cost of Paint by Type and Brand Tier
Paint prices range dramatically based on quality and brand. The table below provides approximate 2024–2025 retail pricing in the United States.
| Brand tier | Price per gallon | Coverage (sq ft) | Coats needed | Examples |
|---|---|---|---|---|
| Budget | $15–$25 | 250–350 | 2–3 | Glidden, ColorPlace, Valspar (basic) |
| Mid-range | $30–$45 | 350–400 | 2 | Behr Premium Plus, Sherwin-Williams SuperPaint |
| Premium | $50–$75 | 350–400 | 1–2 | Benjamin Moore Regal, SW Emerald, Behr Dynasty |
| Ultra-premium | $75–$120 | 350–400 | 1 | Farrow & Ball, Benjamin Moore Aura, Fine Paints of Europe |
Premium paints often have better pigment density, meaning one coat may suffice where budget paint needs two or three. Over the life of a project, the cost difference can actually be smaller than expected because you buy fewer gallons and spend less time on additional coats.
Labour cost: Professional painters in the US charge $2–$6 per square foot of wall area (including prep and two coats), or $200–$500 per room. DIY saves the labour cost but requires investment in brushes, rollers, tape, drop cloths, and time.
Room-by-Room Estimation Guide
The table below provides quick paint estimates for typical US room sizes, assuming 8-foot ceilings, two coats, standard deductions for doors and windows, and 375 sq ft per gallon coverage.
| Room | Typical dimensions | Wall area (net) | Gallons (2 coats) |
|---|---|---|---|
| Small bedroom | 10 × 10 ft | ~270 sq ft | 1.5 |
| Average bedroom | 12 × 12 ft | ~330 sq ft | 2 |
| Master bedroom | 14 × 16 ft | ~420 sq ft | 2.5 |
| Living room | 16 × 20 ft | ~510 sq ft | 3 |
| Kitchen | 12 × 14 ft | ~280 sq ft (more openings) | 1.5 |
| Bathroom | 8 × 10 ft | ~200 sq ft | 1 |
| Hallway (8 ft wide × 20 ft long) | 8 × 20 ft | ~400 sq ft | 2 |
| Garage (2-car) | 20 × 24 ft | ~640 sq ft | 3.5 |
For a whole-house estimate, sum the individual room values and add 10 % for touch-ups and waste. A typical 2 000 sq ft home with 3 bedrooms, 2 baths, kitchen, living, and dining rooms usually requires 12–15 gallons of wall paint and 2–3 gallons of trim paint.
Pro Painting Tips and Preparation
Professional results depend more on preparation than on the painting itself. Follow these steps for a flawless finish:
Surface preparation:
- Fill nail holes and cracks with lightweight spackle. Sand smooth when dry.
- Clean walls with a damp cloth or TSP (trisodium phosphate) solution to remove grease, dust, and cobwebs.
- Sand glossy surfaces lightly (220-grit) to give the new paint a "tooth" to grip.
- Caulk gaps between trim and walls with paintable latex caulk for clean, sealed lines.
- Remove outlet covers and switch plates. Tape over outlets with painter's tape for protection.
Painting technique:
- Work top to bottom: ceiling first, then walls, then trim and doors.
- "Cut in" with a 2.5-inch angled brush along edges, corners, and trim before rolling.
- Roll in a "W" pattern, then fill in with even, overlapping strokes to avoid lap marks.
- Maintain a wet edge — do not let a partially rolled section dry before blending into the next section.
- Allow proper drying between coats: 2–4 hours for latex, 24 hours for oil-based.
Common mistakes to avoid:
- Skipping primer on new drywall or over dark colours — this causes bleed-through and uneven colour.
- Overloading the roller — leads to drips and heavy texture.
- Painting in direct sunlight or extreme heat — causes the paint to dry too fast, leaving brush and roller marks.
- Using cheap roller covers — they shed fibres into the paint and produce a rough finish. Use quality microfiber or woven covers.
VOC Regulations and Environmental Considerations
Volatile organic compounds (VOCs) in paint contribute to indoor air pollution and smog. Environmental regulations limit VOC content, and these limits vary by region.
| Region / Standard | Flat paint limit (g/L) | Non-flat paint limit (g/L) |
|---|---|---|
| US EPA (national) | 250 | 380 |
| California (SCAQMD Rule 1113) | 50 | 100 |
| OTC (Northeast states) | 100 | 150 |
| LEED / Green Seal GS-11 | 50 | 150 |
| EU Directive 2004/42/EC | 30 | 100 |
Low-VOC (< 50 g/L) and zero-VOC paints are now widely available from all major brands without sacrificing performance. If painting nurseries, bedrooms, or spaces for people with respiratory sensitivities, choose zero-VOC products and ventilate well during and after application.
Lead paint warning: Homes built before 1978 may contain lead-based paint. Disturbing lead paint (sanding, scraping) without proper containment is a federal health hazard. EPA's Renovation, Repair, and Painting (RRP) Rule requires certified contractors for work in pre-1978 homes. DIYers should follow EPA guidelines, use HEPA-filtered equipment, and consider professional testing before disturbing old paint.
Exterior Paint Coverage and Considerations
Exterior paint estimation follows the same area-based method as interior painting, but with important differences in coverage rates, surface conditions, and weather constraints.
Exterior coverage rates:
| Surface | Coverage per gallon (sq ft) | Notes |
|---|---|---|
| Smooth wood siding (painted) | 350–400 | Previously painted, good condition |
| Rough-sawn or weathered wood | 200–300 | Absorbs significantly more paint |
| Vinyl or aluminium siding | 300–400 | Must use exterior acrylic latex; no oil-based |
| Stucco | 150–250 | Highly textured; use elastomeric or masonry paint |
| Brick | 100–200 | Extremely porous; primer essential |
| Concrete / block foundation | 150–250 | Use masonry-specific paint |
| Trim and shutters | 300–400 | Semi-gloss or gloss for durability |
Weather requirements: Most exterior latex paints require application when the temperature is between 50 °F and 85 °F (10–30 °C), with no rain expected for at least 4–6 hours. Humidity should be below 85 %. Avoid painting in direct afternoon sun — the surface temperature of dark-coloured siding can exceed 100 °F even when the air is 75 °F, causing the paint to dry too quickly and leaving brush marks. Early morning, late afternoon, or overcast days provide the best conditions.
Exterior surface prep: Power-wash the surface to remove dirt, mildew, and loose paint. Allow 24–48 hours for the siding to dry completely. Scrape and sand any peeling areas down to a firm edge. Spot-prime bare wood with an exterior primer. Caulk gaps around windows, doors, and trim with paintable exterior caulk. This prep work is time-consuming but determines how long the paint job lasts — a well-prepped exterior paint job lasts 7–10 years, while a poorly prepped job may peel within 2–3 years.
Frequently Asked Questions
How many coats of paint do I need?
Two coats is standard for most paint jobs. One coat may suffice when repainting the same colour with a premium paint. Three coats may be needed when going from a very dark colour to white, or when using budget paint with low pigment density. Primer counts as a coat if using a paint-and-primer product.
Latex vs oil-based paint: which should I use?
Latex (water-based): easy cleanup with soap and water, fast drying, low VOC, flexible — best for walls, ceilings, and most interior surfaces. Oil-based (alkyd): harder, more durable, slower drying, strong odour, requires mineral spirits for cleanup — best for high-wear trim, cabinets, metal, and exterior wood. Many jurisdictions now restrict oil-based paint due to VOC regulations.
How do I calculate ceiling paint separately?
Measure the room's length and width: Length × Width = ceiling area in square feet. A 12 × 14 ft room has a 168 sq ft ceiling. Divide by coverage (350–400 sq ft/gallon) = 0.4–0.5 gallons per coat. One gallon is usually plenty for an average room ceiling with two coats. Use flat or matte finish on ceilings to hide imperfections and reduce glare.
How much does it cost to paint a room?
DIY: $50–$150 per room (paint, brushes, roller, tape, drop cloth). Professional: $200–$500 per room depending on size, height, and prep work. A whole-house interior (2 000 sq ft) runs $2 000–$5 000 professionally or $400–$800 DIY for materials.
Can I paint over wallpaper?
It is possible but not recommended. Paint can cause the wallpaper adhesive to release, creating bubbles. If you must paint over wallpaper, ensure all seams are firmly adhered, apply an oil-based primer (to avoid reactivating the paste), and use two topcoats. The better approach is to remove the wallpaper, skim-coat the walls if needed, prime, and paint.
How long does paint last in the can?
Unopened latex paint lasts up to 10 years if stored in a temperature-controlled space. Once opened, latex paint typically lasts 2–5 years if the lid is sealed tightly. Oil-based paint lasts up to 15 years unopened. If the paint smells sour or has lumps that cannot be strained out, discard it. Always stir thoroughly before using stored paint.
What is the best roller nap for walls?
Smooth walls: 3/8-inch nap. Light texture (eggshell, orange peel): 1/2-inch nap. Medium texture (knockdown): 3/4-inch nap. Heavy texture (popcorn ceilings): 1-inch or 1-1/4-inch nap. Using too short a nap on textured walls leaves holidays (unpainted spots); too long a nap on smooth walls creates unwanted texture.
Do I need to sand between coats?
Not usually for walls. If the first coat has drips, brush marks, or nibs (small bumps from debris), lightly sand with 220-grit sandpaper and wipe with a tack cloth before the second coat. For trim and cabinets, light sanding between coats gives the best finish. Always sand oil-based coats before adding another layer.
How much extra paint should I buy?
Buy 10–15 % more than your calculation indicates. Keep at least one quart of the final colour for future touch-ups. Paint formulas can be re-mixed by the store, but pigment batches shift slightly over time — an original leftover from the same batch guarantees a perfect match.
Is paint-and-primer-in-one worth it?
Paint-and-primer products contain more resin and pigment, offering better adhesion and coverage than standard paint. They work well for repainting same-colour or similar-colour surfaces. However, for bare drywall, stain blocking, or dramatic colour changes, a dedicated primer followed by topcoat still outperforms paint-and-primer products.