Area Calculator – Calculate Rectangle, Triangle & Circle Areas

Calculate the area of rectangles, triangles, and circles using metric or imperial measurements. Enter your dimensions above to get the result, the formula used, and the step-by-step calculation.

No sign-up, no ads interrupting your calculation – just enter your dimensions and get an accurate area calculation with the formula and step-by-step calculation shown.

👉 Select your shape above, enter your measurements, and calculate.

What is Area? – Explained Simply

Area measures how much two-dimensional space a shape covers. Think of it as the amount of paint needed to cover a surface, or the number of floor tiles required to fill a room with no gaps. Area is expressed in units such as square meters (m²), square feet (ft²), square centimeters (cm²), acres, or hectares.

This is different from perimeter (the distance around a shape's edge) and volume (space in three dimensions). A common mix-up: doubling the length of a square's side doesn't double its area – it quadruples it. More on that below.

How to Use the Calculator

1. Select your shape – Rectangle, Triangle, or Circle from the tabs above.
2. Choose your unit – meters, centimeters, kilometers, inches, feet, yards, or miles.
3. Enter your measurements – length and width, the three side lengths, or the radius, depending on the shape.
4. Click Calculate – get your area instantly, along with the formula and step-by-step math.

Common Area Calculations

Calculate the area of a rectangle

This is the calculation most people need when they Google "how many square feet is my room" or "rectangle area calculator." You know the length and width – the calculator gives you the area with one click.

Also searched as: "square footage calculator," "room area calculator," "length times width calculator."

• Flooring a room – "How many square feet is a 12 ft by 15 ft bedroom?"
• Buying sod or turf – "Area of a 20m by 10m backyard"
• Working out how much wall area you need to paint – "Wall area for a 4m by 2.5m wall"
• Building a patio or deck – "Area of a 10 ft by 12 ft deck"
• Renting storage or office space – "Square footage of a 30 ft by 40 ft warehouse"

Note: knowing the wall area is the first step toward estimating paint – you'll still need to check your paint's coverage rate per liter/gallon and how many coats you're applying before buying.

Calculate the area of a triangle

Used when you know all three side lengths and need the enclosed area – common in construction, land surveying, and geometry homework. This calculator uses Heron's formula, which works from the three side lengths alone – no need to measure height separately. Frequently searched as "triangle area calculator" or "area of a triangle with 3 sides."

Also searched as: "Heron's formula calculator," "triangle area from sides."

• Land plots – "Area of a triangular lot with sides 30m, 40m, 50m"
• Roof sections – "Triangular roof gable area from three edge measurements"
• School and exam problems – "Find the area of a triangle with sides 5, 6, and 7 cm"
• DIY and fabric cutting – "Area of a triangular piece of fabric or wood"

If you only know the base and height instead of three sides, the area is simply ½ × base × height – see the formula section below.

Calculate the area of a circle

For circular tables, pools, pipes, gardens, or any round object. People search this as "circle area calculator" or "area of a circle from radius."

Also searched as: "radius to area calculator," "round area calculator," "pi r squared calculator."

• Garden design – "Area of a circular flower bed with a 2m radius"
• Pools and hot tubs – "Surface area of a round pool, 4m diameter"
• Pizza and food comparisons – "Area of a 12-inch pizza vs a 16-inch pizza"
• Pipes and tanks – "Cross-sectional area of a pipe with 10cm radius"

Note: the calculator takes a radius, not a diameter. If you only have the diameter, divide it by 2 first.

A Practical Example: From Blueprint to Flooring

Here's how these formulas work together in a real-world scenario.

Tom is renovating his living room, which measures 6m by 4.5m.

• Step 1 – Calculate the room's area (rectangle formula):
  Length × Width = Area
  6m × 4.5m = 27 m²
• Step 2 – Add extra for waste and cuts. A common starting allowance is around 10%, though the right amount depends on the room's shape, the flooring pattern, and the material:
  27 m² + (27 m² × 10 ÷ 100) = 27 m² + 2.7 m² = 29.7 m²
  This is roughly how much flooring material Tom should plan to buy – his supplier can confirm the right margin for his specific material.
• Step 3 – His flooring comes in boxes covering 2 m² each. How many boxes does he need?
  29.7 m² ÷ 2 m² per box = 14.85 → round up to 15 boxes
• Step 4 – The room has a triangular reading nook in one corner, with sides of 3m, 4m, and 5m. What's its area? Using Heron's formula:
  s = (3 + 4 + 5) ÷ 2 = 6
Area = √[6(6-3)(6-4)(6-5)] = √[6 × 3 × 2 × 1] = √36 = 6 m²
• Step 5 – A round rug will sit in the center, with a 1m radius. What area does it cover?
  Area = π × r² = π × 1² ≈ 3.14 m²
• Step 6 – If the flooring is on sale, use the Percentage Calculator to calculate the discount before buying.

Use the calculator above to work out the area for each shape – rectangle, triangle, and circle – then apply these steps to your own project.

The Formulas Behind the Area Calculator

• Formula 1: Rectangle
  Length × Width = Area
• Formula 2: Triangle – three known sides (Heron's Formula, used by this calculator)
  s = (a + b + c) ÷ 2
Area = √[s(s−a)(s−b)(s−c)]
• Formula 3: Triangle – base and height (if that's what you have)
  Area = ½ × base × height
• Formula 4: Circle
  Area = π × r² (where r = radius; if you have the diameter, divide it by 2 first)

Area Calculations Work Everywhere – With All Units

Area formulas are unit-independent. Whether you're working in meters, feet, or inches, the formula stays the same – only the unit label changes.

• Rectangle in meters: 6m × 4.5m = 27 m²
• Rectangle in feet: 20ft × 15ft = 300 ft²
• Circle in centimeters: π × (10cm)² ≈ 314 cm²
• Circle in inches: π × (5in)² ≈ 78.5 in²

Make sure both measurements are in the same unit before calculating – mixing meters and centimeters in one formula is one of the most common mistakes (see below). The calculator handles unit selection for you, but it's worth knowing if you're checking the math by hand.

Common Mistakes in Area Calculations

• Doubling a side doesn't double the area. A 2m × 2m square has an area of 4 m². Double each side to 4m × 4m, and the area becomes 16 m² – four times larger, not two.
• Mixing units before calculating. Multiplying a length in meters by a width in centimeters gives a meaningless result. Always convert both measurements to the same unit first.
• Confusing radius and diameter in circle formulas. The area formula uses the radius, not the diameter. If you're only given the diameter, divide it by 2 before calculating.
• Forgetting that units get squared during conversion. 1 meter = 100 cm, but 1 m² ≠ 100 cm² – it's actually 10,000 cm² (100 × 100). Always square the conversion factor, not just the number.
• Confusing area with perimeter. Area measures surface coverage (m²); perimeter measures the distance around the edge (m). A room's area tells you how much flooring you need – its perimeter tells you how much baseboard trim you need. They aren't interchangeable.
• Assuming area alone tells you how much paint or material to buy. Area is the starting point, but paint quantities also depend on coverage rate per liter/gallon and number of coats. Flooring quantities depend on waste allowance and pattern. Use the area as your base number, then check your specific product's coverage guidance.

FAQs (Frequently Asked Questions)

Related Calculators and Tools

• Percentage Calculator – for discounts, VAT, and percentage changes on your project costs
• Fraction Calculator – for all types of fraction calculations
• Math / Decimal Calculator
• Discrete Mathematics

Last updated: August 2026