Drain Tile Size Calculator
Calculate the required drain tile pipe diameter from drainage area, drainage coefficient, and pipe slope using Manning's equation for agricultural tile design.
Terrace Spacing Calculator
Calculate agricultural terrace spacing from land slope, soil type, crop type, and management practice. Reduce erosion with properly spaced terraces.
%
Vertical Interval⧉
5.0 ft
Horizontal Interval⧉
83 ft
Terraces per 1000 ft⧉
12.0
Coefficients⧉
a=0.6, b=1.4
For selected conditions
Planning notes, formulas, and examples
About the Terrace Spacing Calculator
Terraces are earthen ridges built across slopes to intercept runoff, reduce sheet and rill erosion, and channel water safely to outlets. Proper spacing (horizontal interval) depends on the slope steepness, soil erodibility, crop type, and management practices.
The vertical interval (VI) method is the traditional approach: VI = aS + b, where S is the land slope in percent and a and b are coefficients based on soil, crop, and practice. The horizontal interval (HI) is then derived from the VI and slope.
This calculator uses the NRCS-based vertical interval formula to determine terrace spacing for your specific field conditions, giving both the vertical and horizontal intervals for layout purposes. Use this page to estimate terrace intervals before staking the field or comparing land loss against erosion control benefits.
When This Page Helps
Terraces are one of the most effective erosion control practices. This page helps balance erosion reduction against loss of farmable area before terrace layout is finalized.
How to Use the Inputs
- Enter the land slope as a percentage.
- Select the soil erodibility class.
- Select the crop and management practice.
- Read the vertical interval (VI) and horizontal interval (HI).
- Lay out terraces in the field at the computed horizontal spacing.
Formula used
VI (ft) = (a × S) + b
HI (ft) = VI / (S / 100)
Where:
S = field slope (%)
a = slope factor (varies by soil and practice)
b = base interval (varies by practice)Example Calculation
Result: VI = 5.0 ft; HI = 83 ft
With a = 0.6 and b = 1.4 for moderate erodibility with contour cropping: VI = 0.6 × 6 + 1.4 = 5.0 ft. HI = 5.0 / 0.06 = 83.3 ft. Terraces should be built approximately 83 ft apart horizontally.
Tips & Best Practices
- Common VI range: 3–8 ft depending on slope and conditions.
- HI decreases on steeper slopes (terraces are closer together).
- Steeper slopes with erodible soils need closer spacing.
- Terraces must outlet to a stable waterway or tile inlet.
- Broad-base terraces can be farmed over; narrow-base cannot.
- Consult your local NRCS office for state-specific terrace design standards.
Terrace Layout in the Field
Start at the top of the slope and work down. Use a laser level or survey transit to mark the first terrace line at the computed VI below the ridge. Each successive terrace is located one VI lower. Adjust terrace lines to follow contours and avoid sharp S-curves that complicate farming.
Terrace Outlets
Every terrace must outlet to a stable conveyance: a grassed waterway, tile inlet (conservation practice called “underground outlet”), or an earthen channel. Inadequate outlets cause terrace overtopping and failure.
Terraces and No-Till
No-till farming reduces runoff and erosion, which may allow wider terrace spacing. Some fields with well-managed no-till and cover crops can reduce VI by 10–20%, potentially eliminating the need for one or more terrace lines on moderate slopes.
Sources & Methodology
Last updated:
Frequently Asked Questions
The vertical interval (VI) is the vertical height difference between successive terraces. It determines how much slope length is protected between each terrace. VI is used to calculate the horizontal spacing.
The horizontal interval (HI) is the map (horizontal) distance between terraces. It equals VI divided by the decimal slope. This is the number you measure in the field for layout.
They come from NRCS state design manuals and depend on soil erodibility (K factor), crop cover, and conservation practice. Values of a range from 0.4 to 0.8; b from 0.5 to 2.0.
Broad-base (farmable), narrow-base (grass only), steep-backslope, and bench terraces. Broad-base on gentle to moderate slopes is most common in the Midwest; steep-backslope for steeper terrain.
Yes. Annual inspection after heavy rains. Repair any breaches, low spots, or rodent damage. Maintain outlets (inlets, waterways) in good condition. Avoid tillage that moves soil into the terrace channel.
Yes. NRCS EQIP (Environmental Quality Incentives Program) typically covers 50–75% of terrace construction cost. Contact your local NRCS or Soil and Water Conservation District office.
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