Yield on cost is the income a holding paid over the trailing twelve months divided by the cost basis of the shares you still own. Current yield divides the same income by the position's market value instead. Both describe the same payments; they differ only in what they measure those payments against.
Why the two diverge
The cost basis of a position is fixed at what you paid. Its market value is not. A company that raises its dividend while its share price also rises can leave current yield roughly unchanged while yield on cost climbs year after year, because the denominator in one figure moves and the denominator in the other does not.
That divergence is the whole reason both figures exist. Current yield answers what the shares would pay someone buying today. Yield on cost answers what your own purchase is paying you now. Neither is a better number — they answer different questions, and a position can look attractive on one and unremarkable on the other.
What yield on cost does not say
A high yield on cost is a fact about a purchase price that may be years old. It says nothing about whether the holding is worth keeping, whether the payment is sustainable, or how the position has done overall — a dividend paid out of a shrinking business still raises the figure. It also says nothing about total return, which combines income with the price move.
Comparing yield on cost across two people holding the same security is meaningless: the difference is entirely in when each of them bought.
Reinvestment
Reinvesting a dividend buys more shares, which adds to cost basis. The income and the purchase are two separate events, and recording them as two is what keeps the reinvested shares from being counted as both a payment and a new contribution.
How NetWorthy computes it
NetWorthy sums the dividends you have recorded against a holding in the trailing 365 days, then divides by the cost basis you entered for yield on cost and by the current market value for current yield. Both read as blank rather than 0% when the figure they divide by has not been recorded, and the income also folds into the holding's total return so the return figure is not price-only.
See also Cost Basis and Unrealized Gain.