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The four curves, and who each one helps
A curve is not one thing. Every method reshapes a class differently, and the choice determines which students benefit, so it is worth knowing what you are choosing before you apply it to real marks.
Adding a flat number of points is the honest, transparent one. Everybody gains the same amount, the spread of the class does not change, and any student can check the arithmetic themselves. Its weakness is at the top, where papers already in the nineties hit the ceiling and gain less than everyone else, and at the bottom, where a paper that scored eleven is still failing after five points.
Scaling so the highest paper becomes 100 multiplies every score by the same factor. This one rewards the strong most in raw points: if the top paper was an 80, the factor is 1.25, so an 80 gains twenty points and a 40 gains only ten. It also hands enormous power to a single student, because the whole class is anchored to one paper. One unusually strong result compresses everybody else.
The square root curve takes the root of each score and multiplies by ten. A 36 becomes a 60, a 64 becomes an 80, an 81 becomes a 90. It lifts weak papers dramatically and strong papers barely at all, which makes it the most generous method at the bottom of a class and the one that compresses the top hardest. Use it when a paper was genuinely too hard rather than when a class was underprepared.
Shifting the class average to a target behaves exactly like the flat curve, except the size of the shift is worked out from the class instead of guessed. It is the right choice when you teach several sections and want them to land in the same place, or when a paper came out harder than the one you set last year.
Reading the table before you commit
The number that matters is not the new average. It is how many letter grades actually moved, and the table highlights exactly those rows. A curve that lifts the average by four points but moves nobody across a boundary has changed nothing that appears on a transcript, and the calculator says so plainly when that happens.
The result card also reports how many papers pass before and after. If your intention was to rescue students who were close to the line, that figure is the one to check, and it is common for a curve to move the average handsomely while rescuing nobody, because the failing papers were further below the line than the curve reaches.
Everything on this page uses the standard ten point letter scale, where ninety is an A and sixty is a pass. If your school marks on a different scale, the test grade calculator handles the seven point and plus-and-minus scales for individual papers.
When a curve is the wrong tool
Curving is a way of correcting an assessment, not a way of correcting a class. If the scores are low because the paper contained an ambiguous question or covered material that was never taught, the cleanest fix is to drop that question and rescore, which is defensible to anybody who asks. A curve applied on top of a genuinely fair paper just moves the numbers and tells students their marks are negotiable.
It is also worth saying that curving downward, or grading strictly on a fixed distribution so that a set proportion of the class must fail regardless of what they scored, has fallen out of favour for good reason. It puts students in competition with each other rather than with the material, and a strong cohort is punished for being strong. Nothing on this page will lower a score, which is deliberate.
Whatever you choose, tell the class what you did and why before you release the marks. A curve that arrives unexplained reads as arbitrary even when it was generous, and the explanation costs you one sentence.