Estimating lengths
A tailor doesn't always reach for the measuring tape. A carpenter doesn't always stop and check. With practice, the eye becomes a measuring tool. This is called estimation , a smart guess.
What is this?
Estimation is about having reference lengths in your head. If you know that your hand-span (from thumb to little finger when stretched) is about 18 cm, you can measure a notebook in hand-spans and then convert.
Some useful body references:
- A fingernail is about 1 cm wide.
- A thumb (from tip to first knuckle) is about 3 cm.
- A hand-span is about 18–22 cm.
- A foot length is about 20–25 cm.
- A long step (one walking pace) is about 60–70 cm.
- Both arms stretched out is about your own height , a metre or more.
For longer distances:
- 100 steps ≈ 60–70 m.
- 1 km ≈ 1,200 to 1,500 steps for a child.
Estimation has two parts: pick a unit and pick a number. "About 5 m" is a good estimate. "5 m 23 cm" is not an estimate , it is a measurement.
We always check our estimate by measuring later. With time, our guesses get closer and closer to the truth.
Worked examples
Example 1. Estimate the width of your dining table without measuring.
- Use your hand-span (~20 cm).
- Suppose 5 hand-spans fit across.
- Estimate: 5 × 20 = 100 cm = 1 m.
Example 2. Estimate the height of your front door.
- An average door is about 2 m tall.
- Most adults can just touch the top with a fingertip → height ≈ 2 m.
Example 3. Estimate the length of your room.
- A long walking step ≈ 70 cm.
- Suppose 6 steps fit across.
- 6 × 70 = 420 cm = about 4 m.
Try it
- Estimate the length of your maths book in cm.
- Estimate the distance from your front door to the gate in metres.
- Without a ruler, draw a 10 cm line. Then measure to see how close.
- Estimate the length of a cricket bat in cm.
- Estimate the width of a road in metres.
Hands-on
Span the room. Use your hand-span to measure the length and width of one room. Convert hand-spans to cm or m at the end. Then compare with a tape measure.
Step estimate. Walk from one end of your house to the other. Count the steps. Multiply by your usual step length to estimate the distance in metres.