How to Convert Celsius to Fahrenheit Without a Calculator

Temperature conversion can feel tricky, especially if you don’t have a calculator on hand. But the good news is that converting Celsius (°C) to Fahrenheit (°F) is simple once you understand the formula. With a little practice, you can even do it in your head.

The Official Formula

To convert Celsius to Fahrenheit, the exact formula is:

°F = (°C × 9/5) + 32

This means you multiply the Celsius value by 9, divide it by 5, and then add 32.

Example:
25°C → (25 × 9/5) + 32 = (45) + 32 = 77°F

This method works every time, but if you want something faster for mental math, keep reading.

Quick Mental Math Trick

If you don’t need exact precision, you can use a simple shortcut:

Multiply the Celsius value by 2, then add 30.

It won’t be 100% accurate, but it’s close enough for everyday use.

Example:
20°C → (20 × 2) + 30 = 70°F (actual: 68°F)
30°C → (30 × 2) + 30 = 90°F (actual: 86°F)

As you can see, the difference is only a few degrees, which is good enough for weather forecasts or casual conversations.

Step-by-Step Guide

  1. Start with the Celsius temperature you want to convert.
  2. Multiply by 9 (or by 2 for the shortcut).
  3. Divide by 5 (skip this step if you’re using the shortcut).
  4. Add 32 to get the Fahrenheit value.

Why Add 32?

The Celsius and Fahrenheit scales don’t start at the same zero point. Water freezes at 0°C but at 32°F. That’s why you always add 32 in the formula—it adjusts for the offset between the two scales.

When to Use the Exact Formula vs. Shortcut

  • Exact formula: Use when accuracy matters, such as cooking, science, or schoolwork.
  • Shortcut formula: Use for quick weather conversions, casual travel, or estimating temperatures.

Celsius to Fahrenheits FAQs

Multiply by 2 and add 30 for a quick estimate. For accuracy, use the full formula.

Because Celsius and Fahrenheit have different zero points and scale divisions this formula aligns the two.

Yes. The reverse formula is: °C = (°F − 32) × 5/9.

Subtract 30 from the Fahrenheit value, then divide by 2.