One sachet would do quite a bit. It goes by weight, and the box tells you how dark/true the final colour will be for what weight, IYSWIM. I've done jeans with excellent results. I've also done kids cotton school cardigans half way through the year, when they've faded. They look better than when new! Most things are a v good success. I've done the dress thing but the flipping polyester thread made it a failure.