In APA format...
Use numerals to express:
a. numbers 10 and above
examples: 12 years old, the 57th trial, 12 cm wide
b. numbers that precede a unit of measurement
examples: 5-mg dose, 36.3 mm
c. numbers that represent statistical or mathematical functions, fractional or decimal quantities, percentages, and ratios
examples: multipled by 5, .33 of the..., more than 5% of the sample..., a ratio of 15:1
d. numbers that represent time, dates, ages, scores, points on a scale, exact sums of money and numerals
examples: 1 hr 34 min., at 3:45 am, 2-year olds, score 5 on a 12 point scale
Use numbers expressed as words:
a. when the number begins a sentence, title, or heading
examples: Forty-eight percent of the sample..., Twelve students improved...
b. common fractions
examples: one fifth of the class..., two-thirds majority
c. universally accepted language
examples: the Twelve Apostles, Five Pillars of Islam
For more information, see the Publication Manual of the American Psychological Association.