One-Third Mowing Rule Calculator
Enter the height you mow to and how fast the grass is growing. The calculator gives the height at which a mowing removes exactly one-third of the blade and how many days the grass takes to get there.
Tallest grass for a one-third cut
4.5 in
Growth allowed between mowings
1.5 in
Days between mowings
6 days
Whole days before the limit is passed
6 days
How it works
Extension mowing guidance holds that a single mowing should remove no more than one-third of the leaf blade. Cutting more stresses the grass: UF/IFAS's Mowing Your Florida Lawn calls removing too much shoot tissue at once scalping, and notes the long-term damage it can do.
If the mower cuts to a height T and the grass has grown to H, the cut removes H − T. Keeping that to a third of H means T is at least two-thirds of H, so H can be at most 1.5 × T. UF/IFAS's example: a St. Augustinegrass lawn mowed at 4 in should be mowed before it grows above 6 in.
The grass can therefore grow half the mowing height between cuts. At a steady growth rate, that growth divided by the rate is the number of days between mowings.
Formula
mow-at height = mowing height ÷ (1 − 1/3) = 1.5 × mowing height growth allowed = mow-at height − mowing height = 0.5 × mowing height days between = growth allowed ÷ growth per day whole days = days, rounded down
Example
Mowed at 3 in, the grass can reach 3 × 1.5 = 4.5 in before a mowing takes off more than a third; that cut removes 1.5 in, one-third of 4.5 in.
Growing 0.25 in a day, it gains those 1.5 in in 1.5 ÷ 0.25 = 6 days.
Assumptions and limitations
- Growth is taken as steady. In practice it changes with the grass species, season, temperature, rain or irrigation and fertilizer, so the days figure is an average for the rate entered. The default rate is illustrative, not a measured figure.
- Recommended mowing heights vary by species, cultivar, season and region; the height entered is your figure. For example, UF/IFAS Mowing Your Florida Lawn (ENH10) lists bermudagrass at 0.5–1.5 in and bahiagrass at 3–4 in; University of Missouri Extension G6705 (revised April 2025) lists summer heights of 3–4 in for tall fescue and 2–3.5 in for Kentucky bluegrass.
- The whole-days figure rounds down, so mowing on that day stays within the one-third limit.
- The rule is a general guideline. It does not cover establishing a new lawn, dormant grass or recovering an overgrown lawn.
Frequently asked questions
Why is the limit 1.5 times the mowing height rather than 1.33 times?
The third is a third of the grass height before the cut, not of the mowing height. Grass 4.5 in tall cut to 3 in loses 1.5 in, which is one-third of 4.5 in. Mowing at 1.33 × 3 = 4 in would remove only 1 in, a quarter of the blade.
What happens when grass grows past the limit?
Cutting it straight back to the usual height removes more than a third of the blade. University of Missouri Extension (G6705) describes cutting a missed lawn halfway back to the intended height first and finishing a couple of days later.
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