Turnover (sales) is frequently used in developing countries as a presumptive income tax base, to economize on the costs of tax administration and taxpayer compliance. We construct a simple model where a size threshold separates firms paying turnover tax from those paying profit tax (regular income tax), and where firms have the option of producing in the untaxed, informal sector. The optimal turnover tax rate trades off two policy concerns: reducing informality and avoiding strategic reductions in sales by firms seeking to remain below the threshold for the profit tax.