Math // Fractions

Decimal place value (age 9+)

Round decimals with two decimal places to the nearest whole number and to one decimal place

Ages
9 to 10
Type
Method
Builds on
1 topic
Leads to
1 topic
Standards
1 code

Quick check

If the learner measures a length as 4.67 m, can they round it to the nearest whole metre — and also to the nearest tenth of a metre?

Ask this to see whether the topic is secure.

Topic // Mastery

What mastery looks like

What a learner can do once this topic is secure.
  • Round 3.47 to the nearest whole number (3) and to 1 d.p. (3.5)
  • Round 12.95 to 1 d.p. (13.0) and explain the boundary case
  • Estimate 4.83 + 2.17 by rounding each to the nearest whole number

Topic // Map

Where this fits

What to learn first and what this topic opens up, with the reason for each link. Required links come first.
  • RequiredMust come first
  • HelpfulHelps, but not essential

Before this

1 topic
  • Decimal place valueRequired

    Fractions // Ages 8 to 9

    Rounding 1dp to whole (Y4) is prerequisite to rounding 2dp to whole or 1dp

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Decimal place value (age 9+)

Ages 9 to 10 // Method

Leads to

1 topic

Topic // Research-informed

Teaching notes

From the Marble Skill Taxonomy. These notes have not been validated in classrooms, so treat them as a starting point for your own judgement.

The approach

Teaches rounding two-decimal-place numbers to the nearest whole and to one decimal place, treating rounding as choosing between two neighbouring values at a chosen precision, decided by the single digit immediately to the right of the target place. Two central traps: double rounding (3.47 → 3.5 → 4 when the nearest whole is 3) and the boundary carry (12.95 to 1dp → 13.0, where the rounded tenth ripples into the whole). Rounding is linked to estimation (4.83 + 2.17 ≈ 5 + 2 = 7); for this particular pair the rounding errors cancel, so the estimate equals the true sum. A zoomable number line is the lead activity because the same number can be rounded to different precisions and the carry can be watched; a set of checkable questions, a static annotated line, and diagnostic prompts are preserved as complementary tools.

Common misconceptions

  • Double rounding: 3.47 → 3.5 → 4 to the nearest whole
  • Uses the wrong deciding digit — looks at the hundredths (or a chain of digits) when rounding to a whole
  • Drops the boundary carry: 12.95 → 12.10 or 12.9 instead of 13.0
  • Omits the trailing zero / place: writes 13 when 1dp is asked, or truncates

Useful representations

  • Number line zoomed to the target precision (wholes; then tenths)
  • Place-value chart marking the target place and the deciding digit
  • Estimation on a number line for sums like 4.83 + 2.17
  • Boundary number line 12.9–13.0 for the carry case

Teaching practices

  • Round in ONE step from the original number — never round an already-rounded value; to the nearest whole, only the tenths digit decides (3.47 has 4 tenths, so it rounds to 3)
  • Identify the single deciding digit: the one immediately to the right of the place you are rounding to
  • Handle the carry explicitly on a number line: 12.95 lies between 12.9 and 13.0 and is nearer 13.0, so the whole number changes
  • Keep the target place value: rounding 3.47 to 1dp gives 3.5, and 12.95 to 1dp is written 13.0, not 13
  • Use rounding to estimate and sense-check (4.83 + 2.17 ≈ 7), and judge whether the estimate is above or below

Activities to try

  1. 1

    Zoomable number line: one-step rounding and the visible carry

    Why it works: Round in ONE step from the original number; to the nearest whole only the tenths digit decides, so 3.47 → 3. The boundary carry (12.95 → 13.0) is a real event where a rounded-up tenth ripples into the whole, not a rule failure.

    1. Plot 3.47 on a number line showing whole numbers 3 and 4; mark the 3.5 midpoint and judge that 3.47 sits just below it, so it is nearer 3.
    2. Deliberately try the tempting wrong path: round 3.47 to 3.5 first, then watch 3.5 round up to 4 — the WRONG whole. Compare side by side with going straight from 3.47 (tenths digit 4, below half) which gives 3.
    3. Zoom the line to tenths and round 3.47 to one decimal place (3.5).
    4. Switch to a 12.9–13.0 view and round 12.95 to one decimal place; notice it sits exactly halfway, so by the usual rule (a deciding 5 rounds up) it snaps to 13.0, and watch the whole-number digit tick from 12 to 13 as the carry ripples in.
    5. Use an estimation view to round 4.83 and 2.17 each to the nearest whole and add (5 + 2 = 7); then compare with the true sum (7.00) and say whether the estimate is above, below, or exactly equal — here rounding 4.83 up and 2.17 down cancel, so the estimate equals the true total.
  2. 2

    Identify the deciding digit on a place-value chart

    Why it works: The deciding digit is exactly the one place to the right of the target place (the tenths when rounding to a whole, the hundredths when rounding to 1dp) — never a chain of digits.

    1. Place a number such as 3.47 in a place-value chart and name the place you are rounding to.
    2. Point to the ONE digit immediately to the right of that place as the sole decider before rounding — for rounding to a whole, the tenths digit (4); for rounding to 1dp, the hundredths digit.
    3. Explain why the 7 (hundredths) does NOT decide the nearest whole of 3.47.
  3. 3

    Checkable rounding and estimation questions

    Why it works: A learner can round correctly to a stated precision in one step and use rounding to estimate and sense-check a sum.

    1. Round 3.47 to the nearest whole number (answer 3).
    2. Fill the blank: 12.95 to 1dp = ___ (answer 13.0).
    3. Estimate 4.83 + 2.17 by rounding each to the nearest whole (5 + 2 = 7) and say whether the estimate is above, below, or exactly equal to the true total (7.00) — for this pair the rounding errors cancel, so it is exactly equal.
    4. Round 3.47 to one decimal place (3.5).

Topic // Standards

Curriculum standards

Codes for the curriculum statements this topic maps to. Each publisher owns its standard text.
The national curriculum in England: Key stages 1 and 2 framework document
Ma5/2.4h

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