As rwmj mentions below - avoiding the sums that sanity check the business could be an expensive mistake. Doing the sums is easy (and if you can't make up rough numbers, you ought to ask if you really know your domain). I'm not advocating a full on business plan at the start - just a day spent roughing out the key numbers. Are there enough customers? What do they pay competitors? How much must you sell to break even? Do you know where your customers hang out? That said - if your business has a significant cost of failure (e.g. buying big plant equipment, renting a lease, manufacturing, big design costs etc) then not doing planning is probably rather foolhardy.