Last Updated on October 9, 2026

Garden Lime Calculator: How Much Lime to Raise Soil pH

To raise soil pH one full point, from 5.5 to 6.5, mix about 7.8 pounds of finely ground limestone per 100 square feet into loam, about 6 pounds into sandy loam, and about 10.6 pounds into clay loam. Those numbers come from a University of California Cooperative Extension lime table for the top 7 inches of soil, converted from tons per acre. A 4×8 ft bed of loam needs about 2.5 pounds, split into two applications.

Garden lime calculator

Enter your soil test pH, the pH you want, your soil texture and your bed size. If a soil lab already gave you a lime rate, switch the first box and enter that instead.

How much lime per 100 square feet, by soil texture

This is the table the calculator uses. Soils with more clay or organic matter need more lime for the same change.

Pounds of finely ground limestone per 100 square feet, mixed through the top 7 inches. The source is Table 1 of Changing pH in Soil from University of California Cooperative Extension, which lists tons per acre: 1.7 tons for loam from 5.5 to 6.5, for example. One ton per acre is 2,000 pounds over 43,560 square feet, or 4.59 pounds per 100 square feet, so 1.7 tons is 7.81 pounds. The last column adds the two steps.

Worked example: a 10×10 ft plot of clay loam, pH 4.5 to 6.5

A bigger, heavier job shows the cap and applications logic better than the default. This plot needs the full one-point-plus range and more than one application.

For a partial step, such as 6.0 to 6.5, the calculator takes the same share of the one-point rate: half a point of loam is half of 7.81, or 3.9 pounds per 100 square feet. The source table only gives whole steps, so treat a partial step as an estimate.

How to apply lime, step by step

1. Test the soil first. Cheap soil test kits are unreliable, so send a sample to a lab for the real lime amount (NC State Extension). A lab rate beats this table because the lab also measures buffer pH, which is what decides how much lime you need (University of Maryland Extension). If you have one, use the lab mode above.
2. Lime well before planting. Clemson Extension says lime can go on in fall or winter and works best applied two to three months before planting. For spring crops, count back from your last frost date, which you can look up at Plan My Garden.
3. Spread it evenly, then mix it in. Lime does not dissolve in water, so work it into the top 6 to 8 inches with a tiller or shovel (Iowa State Extension). It reacts slowly in dry soil and needs moisture to work (Clemson Extension).
4. Stay under the per-application cap. Mixed into new soil, you can apply up to 7 pounds per 100 square feet at once (University of Maryland Extension), with the rest about six months later (Penn State Extension). Spread on top of an already planted bed, cap each application at 5 pounds per 100 square feet (Cornell Cooperative Extension of Rockland County). A new bed being tilled can take the whole recommended amount at once (NC State Extension), but the calculator sticks with the two more cautious caps above.
5. Retest before you add more. Wait at least six months and check the pH again before planting perennials (Iowa State Extension). Lime can take 6 months to a year to fully take effect, so an early retest will not show the whole change (UNH Extension).

If plants are already in the bed, Iowa State says to spread the lime on top in the root zone, rake it in as deeply as you can without damaging roots, water well, and split the total into several applications over several years.

Calcitic or dolomitic lime?

Pick by magnesium, not by name. Plain garden lime is calcitic limestone and supplies calcium; dolomitic lime supplies calcium and magnesium and is what a soil lab recommends when pH and magnesium both need to go up (University of Maryland Extension). Calcitic lime can be used on soils already high in magnesium (NC State Extension). If you have no soil test, you have no way to know which you need, which is one more reason to test.

The CCE box handles the strength difference. Penn State Extension explains that calcium carbonate equivalent (CCE) compares a lime to pure calcitic limestone at 100 percent: a 90 percent product is only 90 percent as effective, and its table lists dolomitic limestone at 109 percent. The calculator divides the pounds by your label’s CCE, so a weaker product means more pounds and a stronger one means fewer.

Skip hydrated lime. University of Maryland Extension does not recommend it for gardens because it acts fast and can burn plants if used incorrectly.

Wood ash instead of lime

Wood ash raises pH too, but it takes more of it. UNH Extension says wood ash needs 2 to 4 times as much material as limestone for the same change, though it works faster than lime. The calculator shows that range.

Wood ash is very alkaline, about 10 to 12 on the pH scale, and in my wood ash guide I limit it to 5 pounds per 100 square feet and keep it away from acid-loving plants like blueberries. Do the math on the 4×8 example and that limit is 1.6 pounds, while matching 2.5 pounds of limestone takes 5 to 10 pounds of ash. So ash on its own will not move a bed a full point at that limit. It suits small corrections.

Handle it with care. UNH Extension calls wood ash caustic and says to wear eye protection, gloves and long sleeves, keep it off germinating seeds, and never use ash from garbage, painted or pressure-treated wood.

When the table does not fit your bed

• Raised beds full of compost. The table is for mineral garden soil. Soil high in organic matter resists pH change more (Clemson Extension), and heavily amended garden soil may need its lime buffer capacity measured every year (University of Georgia Extension). For a bed of mostly compost and bagged mix, get a lab rate and use the lab mode.
• Mixing deeper than 7 inches. The same pounds spread through more soil move the pH less. Penn State notes that working lime in deeper than 7 inches raises the amount needed.
• Below pH 4.5, or above 6.5. The table does not go there, so the calculator stops. Get a lab rate.
• Adding extra to be safe. Do not. UF/IFAS Extension warns that too much lime in the soil may be just as bad as too little. To bring pH down, you need a different material and a different calculation.

Quick questions

How much lime does it take to raise pH by 1?

In loam, about 7.8 pounds of finely ground limestone per 100 square feet raises pH from 5.5 to 6.5, mixed into the top 7 inches. Sandy loam needs about 6 pounds and clay loam about 10.6, per a University of California Cooperative Extension table.

How much lime per square foot of garden?

About 0.08 pounds, or 1.25 ounces, per square foot to raise loam from pH 5.5 to 6.5. Sandy soil needs less and clay needs more, so enter your soil texture in the calculator.

How long does lime take to raise soil pH?

Months. UNH Extension says lime can take 6 months to a year to fully take effect. Apply it two to three months before planting (Clemson Extension) and retest about six months later before adding more.

Should I use calcitic or dolomitic lime?

Use dolomitic lime when a soil test shows the soil needs magnesium as well as a higher pH. Otherwise plain calcitic garden lime works, and it is the one to use on soils already high in magnesium.

Can I use wood ash instead of lime?

Yes, for small corrections. Wood ash takes 2 to 4 times as much material as limestone for the same change, and I limit it to 5 pounds per 100 square feet, so it will not move a bed a full point on its own.

Lime the beds, then let the planner fill them

Once the soil is limed, the next job is deciding what goes where. Plan My Garden is free to start: enter your zip code and it finds your frost dates, draw your beds, pick your crops, and it places each plant at square-foot spacing and builds a planting schedule from those dates.

Related: How to Use Wood Ash in the Garden · Raised Bed Soil Calculator · Square Foot Garden Calculator · Charts and Tools