How to tell if a card is worth grading comes down to one subtraction, and the difficulty is entirely in sourcing its three inputs honestly. What the card sells for raw, what it sells for at the grade you will actually get, and what the submission costs you per card once postage is counted.
Input one: the raw price
Five completed sales of the card raw, in your condition, with the highest and lowest dropped. This is the figure you give up by grading, and it is the one people most often set too high, because they price their raw card against sharp examples.
Input two: the graded price at the grade you expect
Not the top grade. Read completed sales at the grade your card will probably receive, which for most raw cards is one or two below what the owner guesses. Because the steps between grades are irregular and the top step is often most of the money, this input decides the answer more than any other.
Input three: the all-in cost per card
The per-card fee for your service level, the extras, and both postage legs divided across the submission. The grading cost calculator produces it. Using the bare fee here instead understates the cost and is the second commonest way a submission turns out badly.
Then subtract, and believe the answer
The uplift calculator applies your marketplace's percentage to both prices, subtracts the cost, and gives the net gain, the break-even graded price and the multiple of raw you need. A negative answer means the card is worth more raw, and that is the correct answer for most cards rather than a reason to try different assumptions.
Questions people ask about how to tell if a card is worth grading
What if I am unsure what grade it will get?
Run the calculation at the grade below your guess as well. If it is still positive there, the submission is robust; if it is only positive at your optimistic grade, it is a gamble.
Does grading help a card sell even if it does not add value?
It usually helps it sell faster and with fewer questions. Whether that is worth the fee is a judgement rather than arithmetic.
Should I grade a card with a visible flaw?
Rarely. A certified low grade is a permanent public record of the flaw, and the card is usually easier to sell raw.
Is bulk grading a way round the arithmetic?
It lowers the fee and the shared postage, which moves the hurdle down. It does not change the logic, and the calculator handles it if you enter the bulk numbers.