Significant Figures in Biology
Throughout secondary school science and in your exams, you will be aked to write values as significant figures. Significant figures (or sig figs) are a way of rounding numbers to get an approximate value. This method accounts for errors that may have been introduced in scientific experiments.
There are a few simple rules that you need to know when doing significant figures in biology, and other sciences. With practice, it will become second nature.
RULE 1: NON-ZERO NUMBERS ARE ALWAYS SIGNIFICANT.
This is the main rule you need to know when rounding numbers to significant figures.
If the number is anything other than zero, i.e. 1 to 9, this is a significant figure. To round your figure to a certain significant number, you’ll need to start counting as soon as you reach that number.
Example 1 – Round 0.728109 to 4s.f
We’ll need to start counting from the first non-zero number which is 7 up until our fourth number; number 1. As we do when rounding decimals, we’ll look at the next number after 1. If the number is five or more, we round up and if it’s four or less we do nothing. The next number is zero. That means our answer is 0.728100 to 4s.f. But we don’t need the last two zeros – they are not significant, so we write 0.7281
Example 2 – Round 0.006274 to 2s.f
Again, we start with the first non-zero number. This number is 6. The second significant figure after that is 2. The number after 2 is five or more. So, we round up and we get 0.006300. Again, we don’t need the last 2 zeros so we can write 0.0063
RULE 2: ZEROS, BETWEEN NON-ZERO DIGITS ARE ALWAYS SIGNIFICANT.
Example 3 – Round 0.02085 to 2s.f
From using the first rule, we know that we need to start counting from the first non-zero number. That number is 2. Then we notice a zero between the 2 and the 8. We’ll need to count this as a significant figure because it’s between two non-zero numbers.
Since the number after 0 is 8 (5 or more), we can therefore round up the second significant figure (the 0).
So, the final number will be 0.02100. We don’t need the last two zeros; they are not significant so we write 0.021 (to 2s.f)
Example 4 – Round 0.02085 to 3s.f
The first sig fig is 2, the second is 0 and the third is 8. The number after 8 is 5 or more. Therefore, we’ll need to round up the number 8.
So, the final number is 0.02090. But we don’t need the last zero so it’s 0.0209
RULE 3: LEADING ZEROS (i.e. zeros before non-zero numbers) ARE NEVER SIGNIFICANT.
Example 5 – Round 0.74593582 to 2s.f
We start counting from number 7 as the leading zero is not significant. So, the first significant figure is 7 and the second is 4. The number after 4 is 5, so we need to round up the number 4.
We get 0.75000000 and we write the answer as 0.75 to 2s.f.
Example 6 – Round 0.006127 to 3s.f
The first significant figure is 6, the second is 1 and the third is 2.
Because the number after 2 is 5 or above, that means we’ll need to round it up. If it was 4 or less, we’ll keep the number the same.
So, we write 0.006130 and then we remove the last zero as it’s not significant. The final number will be 0.00613.
A typical exam question.
Here is an example exam question you might get. This is from the CGP Essential Maths Skills for GCSE Science book.
Tamal carried out an investigation into the rate of photosynthesis using pondweed in a test tube of water. He counted how many bubbles were given off over 1 minute. He repeated the experiment three times and calculated the mean. Give the mean to an approximate number of significant figures.
| Repeat 1 | Repeat 2 | Repeat 3 | Repeat 4 | |
| Number of bubbles produced | 31 | 26 | 35 | 30.6666… |
Your first question might be, how do we know what an ‘approximate’ number of significant figures is? Well, you’ll need to look at the number of significant figures from the question and use that. So, you can see that each result (repeat 1, 2 and 3) are written as 2 significant figures. Therefore, your answer will need to be given as 2 significant figures. It will be 31.
This is a significant figure biology based question but remember, significant figures are popular across the sciences, especially chemistry. So, start practicing so you can get the hang of it. You can download the significant figures worksheet with answers here.
