Quick Answer
To size a farm dam for livestock, work out how many litres your animals need per day, multiply by the number of days you want to cover, then add extra for evaporation and a safety buffer. The result is the storage volume your dam needs (in litres or megalitres), which your contractor can translate into a dam shape and depth.
Intro (plain-English)
If you’ve ever watched your dam level drop faster than you expected, you’re not alone. Most new dam owners underestimate two things: how quickly livestock can drink (especially in hot weather), and how much water you can lose without even noticing it.
This guide is written for beginners. No engineering jargon. By the end, you’ll have a practical way to estimate the dam size you actually need for your stock, plus a few simple checks that stop you building something too small (or paying for something bigger than necessary).
1) Start with the basics: what does “dam size” even mean?
When people say a dam is “big” or “small”, they usually mean how much water it can store, not how wide it looks.
- Storage capacity = the volume of water the dam can hold.
- In Australia, dam storage is often talked about in megalitres (ML).
- 1 ML = 1,000,000 litres.
- Some suppliers also talk in cubic metres (m³).
- 1 m³ = 1,000 litres.
- So 1 ML = 1,000 m³.
If you only take one thing from this post, make it this: You’re sizing storage volume, not surface area. Surface area matters (for evaporation), but storage is the goal.
2) Estimate how much water your livestock need each day
Daily livestock water needs change with:
- temperature
- feed type (dry feed = more drinking)
- lactation (milking animals drink more)
- animal size
Because every farm is different, use these as starter numbers (then confirm with your advisor/Local Land Services or your vet if you want to be precise):
| Livestock type | Typical daily water per head (cool–mild weather) | Hot weather can be more like |
|—|—:|—:|
| Sheep | 2–6 L/day | 6–10+ L/day |
| Goats | 2–6 L/day | 6–10+ L/day |
| Beef cattle | 30–70 L/day | 70–100+ L/day |
| Dairy cattle | 70–120 L/day | 120–150+ L/day |
Your step: pick a realistic daily number for each stock class and multiply by headcount.
Example (simple):
- 80 beef cattle × 60 L/day = 4,800 L/day
3) Choose your “coverage period” (how many days you want the dam to supply)
This is where the dam size can swing wildly.
Ask yourself:
- Is this dam the main water source, or just a backup?
- Do you have a bore, town water, creek pumping, or tanks as a second option?
- How reliable is rainfall where you are?
Beginner-friendly way to pick a number:
- 30–60 days: short buffer (works if you have reliable backup water)
- 90–120 days: decent buffer for many properties
- 180+ days: drought-focused buffer (bigger dam, higher cost)
There’s no “right” answer. The right coverage period is the one that matches your risk tolerance and your backup options.
4) Add losses: evaporation, seepage, and “dead storage”
Even if no livestock drinks a drop, a dam can lose water.
Evaporation (water disappearing into the air)
Evaporation is usually worst when it’s hot, windy, and dry. Bigger surface area = more evaporation.
A practical beginner rule:
- Add 20–40% extra storage to cover evaporation and hot-weather demand.
Seepage (water soaking into the ground)
A well-built clay-lined dam should have low seepage, but many dams leak a bit.
If you’re not sure about your soils (or you’ve had leakage before), be conservative:
- Add another 10–20% or plan for proper sealing during construction.
Dead storage (water you can’t practically use)
The bottom of the dam often ends up:
- muddy
- full of sediment over time
- difficult for stock to access safely
So you generally don’t want to size your dam to “exactly” your calculated need. Build in a buffer.
5) A simple step-by-step sizing method (with worked example)
Here’s a method you can do on a notepad.
Step-by-step
1. Daily demand (L/day) = headcount × litres/head/day
2. Base volume (L) = daily demand × coverage days
3. Add losses & buffer = base volume × (1 + buffer%)
4. Convert to ML: ML = litres ÷ 1,000,000
Worked example
You run:
- 80 beef cattle
- Estimate 60 L/head/day (warm season planning)
- Want 120 days of coverage
- Add 40% buffer for evaporation + uncertainty
1) Daily demand = 80 × 60 = 4,800 L/day
2) Base volume = 4,800 × 120 = 576,000 L
3) Add 40% buffer = 576,000 × 1.4 = 806,400 L
4) Convert to ML = 806,400 ÷ 1,000,000 = 0.81 ML
So you’d plan for roughly 0.8 ML of usable storage for that livestock scenario.
Important note: this is a sizing estimate, not a final design. A contractor will also consider safe batters (side slopes), freeboard (extra height above the waterline), spillway, and local site conditions.
6) Don’t forget the practical side: can your stock actually access the water safely?
Sizing the dam is one thing. Making it usable is another.
Beginner tips that help straight away:
- Keep a firm, gently sloped access point for livestock (to reduce bogging).
- Consider fencing most of the shoreline and using a hardened access area or a trough system.
- Plan for sediment: over years, capacity shrinks. A little extra volume early can save headaches later.
If you’re planning a new dam, talk through access during the design stage. It’s much cheaper to get it right up front.
Key Takeaways
- “Dam size” usually means storage volume, often measured in ML.
- Estimate daily water use per head, then multiply by your chosen coverage period.
- Add a buffer (often 20–40%) for evaporation, hot weather, and uncertainty.
- Build in practical thinking: access, mud, sediment, and leakage risk.
- A local contractor can turn your target storage into a safe dam layout and depth.
FAQ
1) How many megalitres does 100 cattle need?
It depends on daily drinking and how many days you want to cover. As a rough planning example: if 100 beef cattle average 60 L/day each, that’s 6,000 L/day. Over 120 days that’s 720,000 L (0.72 ML), and with a 40% buffer it’s about 1.0 ML.
2) Should I size for average conditions or drought?
If the dam is your main supply, sizing closer to drought conditions (hot weather demand + evaporation) is usually safer. If you have reliable backup water, you may be comfortable sizing for a shorter coverage period.
3) Is a deeper dam always better?
Not always. Deeper dams can reduce evaporation relative to volume, but your site soils, excavation cost, wall design, and spillway requirements matter. A well-designed dam balances depth, footprint, and safe construction.
4) How do I know if my current dam is big enough?
Start by estimating your daily livestock demand, then roughly estimate your dam’s stored volume (a contractor or survey can help). If you regularly run low before reliable rain refills it, it’s usually undersized, leaking, or both.
5) Can Big Ditch help me size and design a dam?
Yes. We can help you plan the right storage for your livestock, your site, and your budget.
Call to action
If you’d like help sizing a new farm dam (or upgrading an existing one) for your livestock and conditions, get in touch here: https://www.bigditch.com.au/contact/
Related Reading
- How long does a farm dam last? And what affects its lifespan
- How Big Is China’s $167 Billion Medog Dam? Scale, Cost, and What It Means for the Rest of Us
- Bentonite clay for dam sealing: how it works and when to use it
About the Author
Lindsey “Angus” Hughson is the founder of Big Ditch Dam Building Co, based at 583 Beranghi Rd, Crescent Head NSW 2440. Angus has built over 341 earth farm dams across New South Wales over 30 years. Before founding Big Ditch, Angus served as President of BHP Diamonds. Call +61 2 7229 4866 or book a site inspection.