size it up.

A little perspective

Trust your eye.

Five comparisons. Two objects each time. How close can you get?

  1. Compare

    The object on the left is your measuring stick. Keep resizing — the view automatically zooms out when you need more room. You can also zoom the canvas; both objects scale together without changing your estimate.

  2. Find the right size

    Drag the blue handle, scroll while hovering it, tap − / +, or use the arrow keys. Match the dimension in the question.

  3. Lock it in

    Reveal the real size and earn up to 100 points. Five rounds add up to a maximum of 500.

The first answer in each round counts. A new challenge arrives every day at 00:00 UTC. Object sizes use the specific examples shown in the answer.

Read the full rules and scoring guide
Practice · not ranked
All past challenges

How long is a tennis ball compared with a golf ball?

Use the golf ball as your reference.

01 / 05
Golf ball, reference object for length comparison
Tennis ball, estimated length relative to the golf ball
Golf ballReference
Tennis ballYour estimate
Fine tune
All past challenges
View answers and sources

Answers from this past challenge. Today’s challenge remains a surprise.

  1. Round 1

    How long is a tennis ball compared with a golf ball?

    Golf ball, used for the length comparison
    ReferenceGolf ball4.267 cm across (minimum)
    Tennis ball, used for the length comparison
    Compared objectTennis ball6.7 cm diameter (representative)

    Illustrations shown individually, not to a shared scale.

    Actual size ratio

    ≈ 1.57×

    Tennis ball ÷ Golf ball, comparing length.

    An ITF Type 2 tennis ball has a diameter of 6.54–6.86 cm. This round uses 6.7 cm, the midpoint, measured across the ball.

    Measurement source · itftennis.com

  2. Round 2

    How long is a humpback whale compared with a sperm whale?

    Sperm whale, used for the length comparison
    ReferenceSperm whale15.8 m long (adult male)
    Humpback whale, used for the length comparison
    Compared objectHumpback whale18 m long (maximum)

    Illustrations shown individually, not to a shared scale.

    Actual size ratio

    ≈ 1.139×

    Humpback whale ÷ Sperm whale, comparing length.

    Humpback whales reach up to about 60 feet (18 metres). This round uses that upper length, not a typical adult.

    Measurement source · fisheries.noaa.gov

  3. Round 3

    How long is a soccer pitch compared with a Titanic?

    RMS Titanic, used for the length comparison
    ReferenceRMS Titanic269 m long
    Soccer pitch, used for the length comparison
    Compared objectSoccer pitch105 m long (international, representative)

    Illustrations shown individually, not to a shared scale.

    Actual size ratio

    ≈ 0.3903×

    Soccer pitch ÷ RMS Titanic, comparing length.

    International matches set the touchline between 100 and 110 metres. This round uses 105 metres, the middle of that range.

    Measurement source · theifab.com

  4. Round 4

    How long is a T. rex compared with a blue whale?

    Blue whale, used for the length comparison
    ReferenceBlue whale33.5 m long (maximum)
    T. rex, used for the length comparison
    Compared objectT. rex12 m long

    Illustrations shown individually, not to a shared scale.

    Actual size ratio

    ≈ 0.3582×

    T. rex ÷ Blue whale, comparing length.

    A typical adult T. rex was about 12 metres long. Fossil specimens vary in size.

    Measurement source · nhm.ac.uk

  5. Round 5

    How long is an orca compared with an American alligator?

    American alligator, used for the length comparison
    ReferenceAmerican alligator3.4 m long (average adult male)
    Orca, used for the length comparison
    Compared objectOrca9.8 m long (maximum)

    Illustrations shown individually, not to a shared scale.

    Actual size ratio

    ≈ 2.882×

    Orca ÷ American alligator, comparing length.

    Killer whales reach up to 32 feet (9.8 metres). This round uses that upper length.

    Measurement source · fisheries.noaa.gov
All past challengesPlay today’s challenge