The UCAS Tariff is a conversion table. It takes a qualification and a grade and turns them into a number, so that a university can compare an applicant with three A levels against one with a BTEC Extended Diploma, or one with Scottish Highers against one with the International Baccalaureate. That is all it is. It is not a ranking of how clever you are, and it is not an offer.
Where the numbers come from
Every qualification on the Tariff is given two things: a size band and a grade band.
The size band reflects how much learning the qualification represents, measured in guided learning hours. An A level is 360 hours and sits in size band 4. An AS level is smaller and sits in band 2. A BTEC Extended Diploma, at 1,080 hours, sits in band 12, three times an A level.
The grade band reflects how demanding each grade is. A levels use grade bands 4 to 14, with an E at 4 and an A* at 14, in steps of two. Other qualifications are mapped on to the same scale, so a Distinction on a BTEC Extended Certificate lands in the same grade band as an A at A level.
Multiply the size band by the grade band and you have the points. An A level grade A is size 4 times grade 12, which gives 48. An A* is 4 times 14, which gives 56. A BTEC Extended Diploma at DDD is 12 times 12, which gives 144.
You do not need to remember any of this to use the tables. It helps to know it, though, because it explains two things people find odd: why a small qualification at a top grade can be worth less than a large one at a middling grade, and why the steps between grades are not always even.
Adding up
Your total is simply the sum of the points for each qualification you hold. Three A levels at B, B and C give 40 plus 40 plus 32, which is 112. A BTEC Extended Diploma at DMM is also 112. Two A levels at B plus an Extended Certificate at Distinction give 40 plus 40 plus 48, which is 128.
There are a few rules about what counts. An AS level does not count if you went on to complete the A level in the same subject. A Scottish Higher does not count alongside the Advanced Higher in the same subject. The International Baccalaureate is scored component by component rather than from the 45-point total. GCSEs are not on the Tariff at all.
What the Tariff is not
The Tariff is not universal. Around a third of university courses express their requirements in points; the rest use grades, and often specific grades in specific subjects. A course that asks for ABB is not asking for 120 points, even though ABB is worth 120 points. Someone with BBB and an EPQ at A* would have 120 points and would not meet that offer.
The Tariff is not a guarantee. A course that asks for 112 points may also require a grade B in one named subject, or a GCSE in maths, or an interview. Meeting the points total is one condition among several.
The Tariff is not complete. UCAS only allocates points to qualifications that have applied for them and been assessed. Plenty of regulated Level 3 qualifications have no Tariff value at all. The OTHM and Qualifi Level 3 diplomas described on this site are in that group: they are real Ofqual-regulated qualifications, they lead to Level 4 study, and they carry no points. If you hold one, or are thinking about one, the calculator will show it as adding nothing, and that is correct.
How to use your number
Work out your total from achieved or predicted grades using the calculator. Then, for each course you are considering, open its entry requirements page and read the whole thing. If it quotes points, compare. If it quotes grades, compare grades and ignore the points. If it says "or equivalent", ask the admissions team what they mean before you assume.
Treat predicted grades honestly. A total built on optimistic predictions will produce offers you then miss, and the time to discover that is now rather than in August.
Finally, if your total is low or zero, read the routes page. Points are one way into higher education, and they are the way that gets the most attention, but Access to HE Diplomas, university foundation years and the Level 3 to Level 4 diploma route all exist for people the Tariff does not serve.