Calculator
Example data table
| Scenario | Total flow (L/min) | Channels | Width (mm) | Slope (%) | n | Root factor | Film depth (mm) |
|---|---|---|---|---|---|---|---|
| Starter greens | 4.0 | 4 | 100 | 1.5 | 0.011 | 1.15 | ~2.0 |
| Heavier root crop | 6.0 | 4 | 100 | 1.5 | 0.011 | 1.35 | ~3.0 |
| Steeper channel | 4.0 | 4 | 100 | 2.5 | 0.011 | 1.15 | ~1.7 |
Example outputs are approximate; your run will reflect your exact inputs.
Formula used
The calculator models shallow flow in a wide rectangular channel using Manning’s equation. For wide sections, the hydraulic radius is close to the water depth, so:
- Q = flow per channel (m³/s)
- w = channel width (m)
- d = film depth (m)
- S = slope (m/m)
- n = roughness coefficient
A root factor is applied to allow extra depth for thicker root mats.
How to use this calculator
- Choose a mode: calculate film depth from flow, or required flow from depth.
- Enter channel count, width, and slope based on your installed layout.
- Set a roughness value that matches your channel material condition.
- Adjust the root factor higher as plants mature and roots thicken.
- Click Calculate to show results above the form instantly.
- Use CSV or PDF export to save a quick report for your records.
Why film depth matters in NFT systems
Nutrient Film Technique relies on a shallow moving layer that keeps roots wet while leaving space for oxygen. When the film is too thin, dry spots can appear along long runs, especially with warm air, wind, or pump ripple. When the film is too deep, the root zone can lose dissolved oxygen and small seedlings may struggle. This calculator helps you balance channel geometry, slope, and flow so depth stays within a practical operating band.
How flow, width, and slope interact
For shallow flow in wide channels, depth increases as flow increases, and decreases as slope increases. Wider channels spread water out, reducing depth for the same flow. Small design changes can make measurable differences, so it is useful to test several scenarios before final plumbing. Splitting total flow across multiple channels also stabilizes distribution and improves repeatability.
Surface condition and roughness effects
Channel surfaces are rarely perfect. New plastic is smoother than aged material, and biofilm can increase friction. The roughness input represents this resistance and influences computed depth. If you notice the same pump setting producing different depths across seasons, clean the channel and re-check leveling.
Root growth allowance and operating margin
Root mats thicken over time and can partially block the flow path. The root factor adds a safety margin so the calculated depth better reflects mature conditions. Use a lower value for young plants and increase gradually as roots expand. Pair depth planning with nutrient temperature control and steady pump operation for best results.
Field checks and calibration workflow
Use a ruler or depth gauge at several points along the channel to confirm actual film depth. Measure after the system reaches steady state, and repeat with plants installed. If measured depth differs from the estimate, adjust the roughness value and verify slope, then save the updated report. Routine checks reduce surprises during hot periods and support consistent crop performance.
- Total flow: 4.0 L/min, Channels: 4, Width: 100 mm
- Slope: 1.5%, Roughness n: 0.011, Root factor: 1.15
- Output film depth: ~2.0 mm, Flow per channel: ~1.0 L/min
FAQs
1) What film depth range is commonly used?
Many NFT setups aim for about 1–3 mm, but crops, temperature, and channel length matter. Use measurements in your system to confirm the practical range.
2) Why does slope change the film depth?
A steeper slope increases driving force and velocity, so the same flow spreads thinner. A flatter slope slows water and increases depth, with higher risk of pooling.
3) How should I choose the roughness value?
Start near 0.010–0.013 for smooth plastic. If channels are aged, dirty, or textured, use a slightly higher value and compare the estimate to a measured depth.
4) What does the root factor represent?
It adds extra allowance for root mats that occupy space and disturb flow. Increase it as plants mature to keep a realistic margin for thicker roots.
5) Can this tool size the pump for my whole system?
Yes, use the required-flow mode to estimate total flow for your target depth. Then add head-loss allowance for lift and fittings when selecting a pump.
6) Why do my channels show uneven depth along the run?
Common causes include leveling issues, partial blockages, air in lines, or inconsistent distribution. Check inlet manifolds, confirm slope, and clean channels before changing targets.
7) How often should I verify depth?
Check at setup, after planting, and after major root growth. In hot seasons or high loads, quick weekly spot checks help catch pump drift or blockages early.