What Is Total Dynamic Head (TDH)? Complete Water Pump Sizing Guide
Learn what Total Dynamic Head (TDH) is, how to calculate it, and why it is essential when choosing a water pump for boreholes, rainwater harvesting, irrigation and domestic water systems.
Understanding Total Dynamic Head (TDH): The Complete Guide
When choosing a water pump, many people focus on motor size or maximum flow rate. While these specifications are important, one of the most critical factors is Total Dynamic Head (TDH).
Total Dynamic Head represents the total amount of energy a pump must provide to move water from the source to its destination. It considers much more than just vertical height—it also accounts for friction losses in the pipework, fittings, filters and the pressure required at the outlet.
Understanding TDH is essential for selecting the right pump. If TDH is underestimated, your pump may struggle to deliver enough water. If it is overestimated, you may purchase a larger, more expensive pump than necessary.
This guide explains what Total Dynamic Head is, how to calculate it, and why it plays such an important role in water system design.
What Is Total Dynamic Head?
Total Dynamic Head is the total resistance that a water pump must overcome to move water through a system.
Think of TDH as the “workload” placed on the pump.
It combines several factors into a single value, usually measured in metres (m).
A pump must overcome:
- Vertical elevation.
- Pipe friction.
- Pressure requirements.
- Resistance from valves and filters.
- Minor losses caused by bends and fittings.
Why Is TDH Important?
Imagine buying a family car because it has a powerful engine. That engine might perform well on flat roads but struggle if it’s towing a heavy trailer up a mountain.
Water pumps work the same way.
Two pumps with identical motors can perform very differently depending on the resistance in the plumbing system.
Knowing your TDH allows you to:
- Choose the correct pump size.
- Improve efficiency.
- Reduce electricity costs.
- Prevent pump failure.
- Extend equipment lifespan.
- Improve water pressure.
The Four Components of Total Dynamic Head
TDH consists of four primary components.
1. Static Head
Static Head is the vertical distance that water must be lifted.
For example:
- Borehole water level = 25 metres below ground.
- Storage tank inlet = 4 metres above ground.
Static Head = 29 metres
This is the easiest part of TDH to measure.
2. Friction Loss
As water flows through pipes, it rubs against the inside walls.
Every metre of pipe creates resistance.
The longer the pipe, the greater the friction loss.
Factors affecting friction include:
- Pipe length.
- Pipe diameter.
- Flow rate.
- Pipe material.
- Water velocity.
Small pipes create significantly more resistance than larger pipes carrying the same amount of water.
3. Pressure Head
Your taps, showers and irrigation systems require pressure.
Pressure is converted into metres of head.
Approximate conversions:
| Pressure | Head |
|---|---|
| 1 bar | 10.2 metres |
| 2 bar | 20.4 metres |
| 3 bar | 30.6 metres |
| 4 bar | 40.8 metres |
If your home requires 3 bar pressure, the pump must provide approximately 30 metres of head before accounting for elevation and friction.
4. Minor Losses
Minor losses are caused by fittings within the plumbing system.
Examples include:
- Elbows.
- Tees.
- Ball valves.
- Non-return valves.
- Filters.
- Pressure reducing valves.
- Flow meters.
Although each fitting creates only a small resistance, together they can significantly increase TDH.
The TDH Formula
A simple way to estimate Total Dynamic Head is:
TDH = Static Head + Friction Loss + Pressure Head + Minor Losses
For example:
- Static Head = 22 m
- Friction Loss = 6 m
- Pressure Head = 30 m
- Minor Losses = 2 m
TDH = 22 + 6 + 30 + 2 = 60 metres
This means you need a pump capable of delivering the required flow rate at 60 metres of head.
Example 1: Rainwater Harvesting System
You have:
- Water tank on ground level.
- Double-storey home.
- Pipe length = 40 metres.
- Desired pressure = 3 bar.
Estimated TDH:
- Static Head = 8 m
- Friction Loss = 4 m
- Pressure Head = 30 m
- Minor Losses = 2 m
Total Dynamic Head = 44 metres
Example 2: Borehole Installation
Your borehole details are:
- Dynamic water level = 38 metres.
- Storage tank = 3 metres above ground.
- Pipe friction = 5 metres.
- Required outlet pressure = 2.5 bar.
Calculation:
Static Head
38 + 3 = 41 m
Pressure Head
2.5 × 10.2 = 25.5 m
Friction
5 m
Minor Losses
2 m
Total
41 + 25.5 + 5 + 2
TDH = 73.5 metres
Example 3: Irrigation System
Suppose you’re irrigating a sports field.
Requirements:
- Flow = 150 L/min
- Sprinkler pressure = 4 bar
- Pipe length = 120 metres
Estimated TDH:
Static Head = 5 m
Pressure Head = 41 m
Friction = 10 m
Minor Losses = 3 m
Total = 59 metres
Static Head vs Dynamic Head
These two terms are often confused.
| Static Head | Dynamic Head |
|---|---|
| Vertical height only | Total resistance |
| Easy to measure | Calculated |
| Doesn’t change much | Changes with flow |
| Ignores friction | Includes friction |
Always size a pump using Total Dynamic Head, not just static height.
Why Pipe Size Matters
Pipe diameter has a significant impact on friction loss.
For the same flow rate:
- Smaller pipes increase resistance.
- Larger pipes reduce friction.
- Lower friction improves pump efficiency.
- Lower friction reduces electricity costs.
Choosing the correct pipe size can sometimes allow you to use a smaller, more efficient pump.
Common TDH Calculation Mistakes
Avoid these common errors:
- Ignoring pipe friction.
- Forgetting filters.
- Excluding pressure requirements.
- Measuring only vertical lift.
- Using maximum pump head instead of duty point.
- Oversizing the pump “just in case.”
- Not considering future system expansion.
Frequently Asked Questions
Is TDH the same as pump head?
TDH is the total head required by the system. Pump head is the amount of head the pump can produce. The pump must be capable of meeting or exceeding the required TDH at your desired flow rate.
Does horizontal pipe count?
Yes. While horizontal pipe does not add elevation, it contributes to friction loss, which forms part of the Total Dynamic Head.
Why does TDH increase when flow increases?
As more water moves through the pipe, friction losses rise. This means the pump must work harder to maintain the same flow.
Can I calculate TDH myself?
Yes, with accurate measurements of elevation, pipe length, fittings and required pressure. However, for larger or more complex systems, professional design software or consultation is recommended.
Conclusion
Total Dynamic Head is one of the most important concepts in pump selection. It reflects the complete resistance that your water system places on the pump, including elevation, pressure, pipe friction and fittings.
By understanding and calculating TDH correctly, you can choose a pump that delivers reliable performance, operates efficiently and avoids unnecessary energy costs or premature failure.
Whether you’re installing a domestic pressure system, designing an irrigation network or fitting a borehole pump, calculating TDH is a critical step toward a successful and efficient water supply system.
Related Articles
Continue exploring our pump and water system guides:
- How to Choose the Right Water Pump
- How to Read Water Pump Performance Curves
- Do You Have a Borehole? Read This Before Buying a Pump
- Pressure Tanks Explained
- Surface Pumps vs Submersible Pumps
- How to Calculate Water Flow Rate
- Common Water Pump Problems and Troubleshooting
- Choosing the Right Pipe Size for Your Water System