Decimal Place Value with Base-Ten Blocks and Number Lines: Build a Decimal

Build a Decimal is a digital place value tool that makes that worth visible: students build decimals with base-ten blocks, and the mat always shows which blocks are whole ones and which are parts of one whole.

It covers tenths, hundredths and thousandths, and every number is shown in words, as a fraction, in expanded form and on a number line as students build it.

Watch it in action

The tool runs in any web browser. There's nothing to print, cut or laminate, and students don't need logins.

Wholes and parts, kept apart

The mat with whole ones on the left and tenths and hundredths filling an outline of one whole on the right

The biggest source of decimal confusion is the whole. With base-ten blocks, a flat might be 100 one day and 1 the next. Build a Decimal settles it on the mat itself:

Wholes sit on the left, under their own heading. In the Tenths and Hundredths versions, a flat is one whole.
Parts of one whole sit on the right, past the decimal point. Tenths and hundredths fill a dashed outline of that same whole, so students see that 10 tenths make a whole and 10 hundredths make a tenth.
The tenths outline has a strong line down the middle, and whole ones sit in rows of five, so counting is quick.
Every piece on the shelf is labelled with its value: 1, 0.1, 0.01 or 0.001.

All the blocks are the same colour, just like the real ones in your classroom. The headings do the sorting, not the colours.

See it four ways

$2.47 shown in words, as a fraction, in expanded form and on a number line$

As soon as a block lands on the mat, the strip underneath updates:

In words: two and forty-seven hundredths
As a fraction: 2 47/100
Expanded form: 2 + 0.4 + 0.07
On a number line: a marker between 2 and 3, a little before 2.5

Students hear the place name in the number ("hundredths"), connect it to the fraction, and see how big the decimal really is.

Three versions

Tenths

Build a Decimal Tenths version: 3 whole ones and 7 tenths filling one whole, for 3.7

Ones and tenths, like 3.7. A great starting point: every tenth is one strip of the whole.

Hundredths

Build a Decimal Hundredths version: 2 ones, 4 tenths and 7 hundredths for 2.47

Ones, tenths and hundredths, like 2.47. Hundredths fill their whole one strip at a time, so ten of them line up exactly with one tenth.

Thousandths

Build a Decimal Thousandths version: the big cube is one whole, with flats as tenths, rods as hundredths and small cubes as thousandths

Ones to thousandths, like 5.862. Here the big cube is one whole, so a flat is a tenth, a rod a hundredth and a small cube a thousandth. It's a great moment to ask: "What is one whole now?"

Each version also has its own link, so you can assign exactly the level you want.

Build it, or name it

Every version has two modes.

Build the number: a decimal appears and students build it with blocks, then check.
Name the number: the blocks show a decimal and students type it. The words, fraction and number line stay hidden until they get it right, then appear as the explanation.

Name the number mode: blocks showing 1 whole and 5 hundredths, waiting for the answer

After a correct answer of 1.05: one and five hundredths, 1 5/100, 1 + 0.05 and the number line

1.2 and 1.20 are both accepted, which makes a great talking point about equivalent decimals. A wrong answer shows what was built, so students can see the difference.

Try again: the target was 4.6 but the blocks show 4.5

Works on any screen

Build a Decimal on a phone screen

The blocks resize to fit any device, from Chromebooks and tablets to the classroom board, so a full mat of nine of every piece always fits without scrolling.

What students practise

Decimal place value to tenths, hundredths and thousandths
Reading and writing decimals in numerals, words and expanded form
Decimal fractions: 0.47 = 47/100
Equivalent decimals: 1.2 = 1.20
Locating decimals on a number line

Teaching tips

Start with the whole. Before building, ask "What is one whole here?" and "How many of these parts make one whole?"
Build 1.2, then 1.20. Same blocks, same amount. Why doesn't the zero change the value?
Read it with the place name. "Two and forty-seven hundredths" makes the fraction 2 47/100 obvious.
Use the number line. Is 2.47 closer to 2 or to 3? Which two tenths is it between?
Talk about the switch. When moving to Thousandths, point out that the whole has changed to the big cube.

Standards

4.NF.C.5 and 4.NF.C.6: decimal notation for fractions with denominators 10 and 100
5.NBT.A.1: place value, each place is 10 times the place to its right
5.NBT.A.3a: read and write decimals to thousandths in numerals, words and expanded form

Quick answers

Do students need accounts? No. It's a link, so there's nothing to sign in to.

What devices does it work on? Any device with a web browser, including Chromebooks, tablets, phones and the classroom board.

Is it a game? It's a self-checking tool: students build or name a number, check it, and get a new one. It works for modelling on the board as well as independent practice.