Many house-flipping professionals use the "70% rule" when they are deciding whether to buy a house to flip. They start with an estimate of the price they think they can sell the house for after it’s renovated. Then they multiple this price by 0.70 or 70% to estimate the price at which they should buy the house now – that is, for a lot less than they plan to sell it. Then, they also subtract their estimate of the cost of the repairs from the purchase price, making this price even lower.