The basic ratio
Almost every student portal uses the same core formula: attendance % = (attended ÷ held) × 100. “Held” is the count of sessions that existed and were marked, not the count printed in the syllabus in week one. If 4 of 48 published lectures were cancelled, your denominator should be 44 unless the office still lists them as held.
This site does not invent a different ratio. The check tab is that formula with a colour next to your chosen target. If attended is greater than held, the tool refuses the input instead of showing more than 100%.
Worked example: mixed session types
Your computer networks paper lists 28 lectures, 10 tutorials, and 6 labs. So far, 20 lectures, 6 tutorials, and 4 labs have been held (30). You missed 1 lecture and 2 labs: attended = 27, percentage = 27/30 = 90%. If the school publishes one blended percentage, you are done. If it publishes three percentages, you must compute each row: lectures 19/20 = 95%, tutorials 6/6 = 100%, labs 2/4 = 50%. A 75% rule on labs alone can fail you even when the blend looks fine. Use the subject tracker to keep those rows separate.
The recovery formula
When you are below target, attending future classes changes both sides of the fraction. Solving (attended + x) / (held + x) ≥ t/100 gives x ≥ (t × held − 100 × attended) / (100 − t), then round up. For 22 attended of 40 held and t = 75: x ≥ (3000 − 2200) / 25 = 32. You need 32 consecutive new classes to reach 54/72 = 75%. That is a different question from “how many of the remaining 10 can I miss”; if only 10 remain, 32 is impossible.
The miss formula asks a third question: given remaining R, is there a number of skips s such that (attended + R − s) / (held + R) ≥ t/100. This calculator ceilings t/100 × (held + R) to an integer requirement, then sets s = attended + R − requirement. Negative s means you are short even with perfect remaining attendance.
Rounding policies differ. Some offices display two decimal places and still require ≥ 75.00. Others integer-truncate. We show one decimal on the check tab and integer miss counts. If your registrar truncates 74.9% to 74%, do not treat 74.6% as safe. Official wording always wins; this page is a working aid. Continue with how many classes can I miss or the 80% attendance calculator if that is your published target.