Estimating products
Free lesson plan from HomeschoolMath.net

a.    158 × 32

≈ 200 × 30 = 6000

c.   29 × 94

≈ ____ × ___ = ____

d.    770 × 33

≈ ____ × ___ = ____

f.  88 × 99

≈ ____ × ___ = ____

j.     486 × 21

≈ ____ × ___ = ____

l.   209 × 27

≈ ____ × ___ = ____

1.    103 × 52

2.    42 × 76

3.    319 × 25

4.    17 × 17

5.    99 × 59

6.    47 × 21

a.  4000

a.  6000

a.  1000

a.  6000

a.  470

b.  500

b.  320

b.  750

b.  200

b.  900

b.  9700

c.  4800

c.  9000

c.  400

c.  9000

c.  1000

Other methods of estimation

When you round both factors to the biggest place value, 249 × 34 would be estimated to 6000; yet the actual product is 8466 - quite far from the estimate.  

• You get a much better estimate if you round 249 to 250, and not to 200.  Round 34 still to 30.  So 249 × 34 ≈ 250 × 30.  To calculate this, first go 3 × 25 = 25 + 25 + 25 = 75, and then tag two zeros to it: 250 × 30 = 30 × 250 = 7500.
  So instead of rounding to nearest hundred, you can round to the 'middle fifty' if the number is close to it.
• Or, even when the number has hundreds, you can round to nearest ten. For example, 178 × 28 is better estimated to 180 × 30 - rounding both numbers to nearest ten.  That you can do mentally by going 100 × 30 + 80 × 30, which is 3000 + 2400 = 5400.
• You can also try to account for the error in your estimation by rounding one factor up, the other down - especially if you have to round one factor a lot.  For example, 144 × 24.  If you round 144 down to 100, round 24 up to 30 to make up for some of the error. 

Estimation is not an exact science but a matter of rounding to close numbers that you can work in your head.