What percentage of the fertilizer is nitrogen in the houseplant fertilizer?

Ten, 15 and 20 percent appear as percentages of nitrogen in houseplant fertilizers. Clemson University extension publications suggest such balanced percentages of nitrogen, phosphorus and potassium (NPK) fertilizers as 10-10-10 and 20-20-20 for houseplants grown for their foliage. They tend toward 15-30-15 NPK percentages for houseplants grown for their flowers.