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Adding Within 100

A 40-minute Grade 1 lesson on adding a one-digit number or a multiple of ten to a two-digit number, where the rule is that you add tens to tens and ones to ones.

Adding Within 100 is a free 40-minute Grade 1 maths lesson plan for Common Core standard 1.NBT.C.4 — add numbers up to 100. It is timed across 5 sections, gives the exact wording to use where the wording carries the mathematics, names 3 misconceptions with the teaching move that addresses each, and ends with an exit ticket and a printable worksheet with its answer key.

Grade:
Grade 1
Length:
40 minutes
Standard:
1.NBT.C.4

Grade 1 · Number & Operations in Base Ten

1.NBT.C.4

Add within 100, including adding a two-digit number and a one-digit number, and adding a two-digit number and a multiple of 10.

Objective

Students will be able to add a one-digit number or a multiple of ten to a two-digit number, regrouping ten ones into a ten when needed.

Students are successful when they can

  • Adds a multiple of ten by changing only the tens digit.
  • Adds a one-digit number and regroups when the ones exceed nine.
  • Explains why the tens digit changed.

What you need

  • Base ten rods and units, or ten frames
  • A 120 chart
  • Place value mats with a tens and a ones column
  • The printable practice sheet linked at the end of this plan

The lesson

Warm-up — ten more

5 min
  1. Say a two-digit number; class says ten more. Do a dozen.
  2. Ask which digit changed each time. Only the tens, every time, and that observation carries the first half of the lesson.

Teach — same kind to same kind

12 min
  1. Build 34 on a place value mat: three rods, four units.
  2. Add 20. Two more rods go in the tens column. Nothing happens in the ones. The answer is 54.
  3. Now add 3 to 34. Three units go in the ones column: 37. Again, one column only.
  4. Now add 8 to 34. Twelve units in the ones column — more than a column can hold.
  5. Trade ten units for one rod in front of them. Four rods, two units: 42. Name the trade rather than calling it 'carrying'.
  6. Ask why the tens digit changed when only ones were added. Because ten ones is a ten, and it moved.
“Tens go with tens and ones go with ones. The only time a column changes on its own is when ten ones have gathered and turned into a ten.”

Guided practice — build and trade

11 min
  1. Pairs build a two-digit number and add a card — either a multiple of ten or a one-digit number.
  2. They must say whether a trade is needed before doing the addition.
  3. Predicting the trade is the skill; performing it is easy once predicted.

Independent practice

9 min
  1. Students complete an adding within 100 page.
  2. Answers where the ones digit exceeds nine — 34 + 8 written as 312 — mean the trade was skipped rather than done wrongly.

Close

3 min
  1. Give 56 + 30 and 56 + 3 back to back and ask what is different.
  2. Different columns, different digits changing. That contrast is the lesson.

What goes wrong, and why

Each of these is a reasoning error rather than carelessness. The fix addresses the reasoning.

Writes 34 + 8 = 312.

Why: The ones have been added correctly to twelve and written in the ones place, because there is no rule yet saying a column can hold only one digit.

Fix: Trade physically. Ten units genuinely will not stay in the ones column of the mat once the rule is that a column holds nine at most, and the trade becomes necessary rather than instructed.

Adds 34 + 20 by counting on twenty ones.

Why: Counting works and place value has not yet offered anything faster. The twenty has been read as a quantity to count rather than as two tens.

Fix: Hand them two rods. Counting twenty units is available and obviously slower, and the pupil chooses the rods.

Adds the digits regardless of column: 34 + 20 = 36.

Why: The 2 in 20 has been treated as a 2, which is what it looks like. Its position has not yet changed what it means.

Fix: Build the 20 rather than writing it. Two rods cannot be confused with two units when both are on the desk.

If they are not there yet

  • Add multiples of ten for a full session before adding ones. Two skills that both change a digit are easier apart than together.
  • Use a 120 chart, where adding ten is one square down and the pattern is visible rather than remembered.

If they finish early

  • Add a two-digit number to a two-digit number where both columns need attention.
  • Add three numbers, choosing an order that avoids a trade.
  • Work out what to add to 47 to reach 60, which reverses the process.

Exit ticket

Work out 45 + 30 and 45 + 7. For each, say which digits changed.

What to look for: 75 with only the tens changed, and 52 with both changed. A pupil who says only the ones changed in the second has done the trade without noticing what it meant.

Printable practice for 1.NBT.C.4

Free to print and copy, answer key included. Each sheet can generate a fresh set of problems for a retake.

Questions about Adding Within 100

How long does the Adding Within 100 lesson take?

40 minutes, split across 5 timed sections: warm-up 5 min, teach 12 min, guided practice 11 min, independent practice 9 min, close 3 min. The minutes are stated per section and add up to the stated length, so the plan can be cut or extended at a section boundary rather than abandoned halfway.

Which standard does Adding Within 100 teach?

CCSS.MATH.CONTENT.1.NBT.C.4 — 1.NBT.C.4, Number & Operations in Base Ten for Grade 1: “Add within 100, including adding a two-digit number and a one-digit number, and adding a two-digit number and a multiple of 10.” In plain terms, add numbers up to 100.

What do I need to teach this lesson?

Base ten rods and units, or ten frames, a 120 chart, place value mats with a tens and a ones column and the printable practice sheet linked at the end of this plan. Everything listed is either ordinary classroom equipment or a free printable from this site — there is nothing to buy and nothing to prepare beyond photocopying.

What mistakes should I expect in this lesson?

Three, each a reasoning error rather than carelessness: writes 34 + 8 = 312; adds 34 + 20 by counting on twenty ones and adds the digits regardless of column: 34 + 20 = 36. The plan gives the thinking behind each one and the teaching move that addresses the thinking rather than the symptom.

How do I know whether the lesson worked?

The exit ticket asks: “Work out 45 + 30 and 45 + 7. For each, say which digits changed.” 75 with only the tens changed, and 52 with both changed. A pupil who says only the ones changed in the second has done the trade without noticing what it meant. That tells you whether the class needs a reteach or a thirty-second conversation before you plan tomorrow.

Is there a worksheet to go with Adding Within 100?

Yes — 2 printable 1.NBT.C.4 worksheets, linked at the end of the plan and free like the rest of the site. Each one comes with an answer key and 25 versions, so the practice after the lesson can be a different paper on every desk.

Is Adding Within 100 free to use?

Yes. The plan, its printables and their answer keys are free to read, print and photocopy for your own class, with no account and no email capture. It was last reviewed on 5 September 2026. Please do not resell it or republish it as your own.

Last reviewed 2026-09-05. Aligned to 1.NBT.C.4 of the Common Core State Standards for mathematics.