Long multiplication calculator

How to use it

1. Type the two numbers. Whole numbers or decimals, up to 30 digits each; a dot or a comma works as the decimal point.
2. Read the working: the carries sit in small rows above the top number, one row per digit of the bottom number and labelled with that digit (×9, ×8, ×2). Each partial product follows, with its shifted zeros in grey, then the carries of the addition and the answer.
3. Untick Show carries to see the layout without them, the way some teachers want it written, or to test yourself.
4. Tap Lattice grid for the same multiplication in the lattice method: one cell per pair of digits and the answer read along the diagonals.
5. For a worksheet, type problems one per line (or tap Add to worksheet), then Make worksheet and Print worksheet. The problems print first, with room to work, and the answers on a page of their own.
6. To check an answer, divide it by one of the numbers: check it by dividing with the long division calculator, which shows every step too.

What it cannot do

• Numbers up to 30 digits each. Anything longer is refused rather than rounded.
• Negative numbers only by the sign rule: the working is drawn without the signs, and the answer is negative when exactly one of the two numbers is negative.
• No area model or box method (the method UK schools call the grid method, which splits 347 into 300 + 40 + 7). Only the column layout and the lattice.
• Two numbers at a time; no exponents such as 1e5, no fractions.
• A worksheet holds up to 24 problems and prints them in the column layout only, not as empty lattice grids.
• Nothing is saved: closing the page forgets your worksheet. Nothing you type is sent anywhere.

Questions

How do you do long multiplication step by step?

Write one number above the other, lined up on the right. Multiply the top number by the ones digit of the bottom number: 347 × 9 = 3123. Then by the tens digit, writing one 0 first because that digit is worth ten times as much: 347 × 8 = 2776, written 27760. Then by the hundreds digit with two zeros: 69400. Add the rows: 3123 + 27760 + 69400 = 100283. The calculator writes each of these as a sentence under the working.

How do you do long multiplication with decimals?

Ignore the decimal points and multiply the digits as whole numbers, then count the decimal places in both numbers together and put the point that many places from the right of the answer. 2.5 × 0.04: 25 × 4 = 100; 2.5 has 1 place and 0.04 has 2, so 3 places: 0.100, which is 0.1. Unlike in addition, the points do not have to line up. A zero you type at the end, as in 2.50, counts as a place, just as it would on paper.

How do you multiply 3-digit numbers?

A 3-digit bottom number gives three partial products, with no zero, one 0 and two zeros on the right, and then one addition. A 3-digit × 3-digit answer always has 5 or 6 digits: 100 × 100 = 10000 and 999 × 999 = 998001, a quick check that no row was shifted the wrong way.

How does carrying work in long multiplication?

When a digit times the multiplier, plus whatever was carried, is 10 or more, write the ones digit and carry the tens to the next column on the left. 347 × 9: 7 × 9 = 63, write 3, carry 6; 4 × 9 = 36, plus 6 is 42, write 2, carry 4; 3 × 9 = 27, plus 4 is 31, write 31. Add the carry after multiplying, not before: (4 + 6) × 9 is a common slip. Each row starts with fresh carries, which is why the calculator gives every row its own carry line.

How do I explain long multiplication to kids?

Start with a 2-digit number times a 1-digit number, then explain the zero: in 289 the 8 means 80, so its row is 347 × 80 and ends in 0. The lattice method keeps the times-table facts and the adding apart, which many children find easier, since every cell is a single fact like 4 × 8 = 32. Put their own homework problems on a worksheet; the answers print on a separate page so they can check themselves.

What is the lattice method of multiplication?

Draw a grid with one column per digit of the first number and one row per digit of the second, and a diagonal through each cell. Each cell holds one digit pair's product, tens above the diagonal and ones below. Add along the diagonals from the bottom right, carrying into the next diagonal, and read the answer down the left and along the bottom. It is also called the gelosia method, and it gives the same answer as the column method: the diagonals are the place values.