In short
- A horse wash rack size and drainage calculator turns your horses, bay size, floor slope and hose use into numbers a builder can quote against: bays, drains, fall, and gallons of wash water.
- Published starting points: 8 x 12 ft minimum, 10 x 12 ft roomier, about 1 inch of fall per 6 ft, and a 50 ft Florida setback from waterbodies, wells and septic drainfields.
- Bays per hour, drain count and gallons per bath are our planning assumptions and are labelled.
- Planning range only. No prices.
On this page
- How the numbers are worked out
- Slope: how much fall over a 12 ft bay
- Wash water by horses per wash day
- Bay size by horse size
- What the sources say
- Notes for an Ocala barn
- What this calculator does not cover
- Questions to ask a builder about the wash rack
- Next step: get a written quote
- Sources
- Frequently asked questions
This horse wash rack size and drainage calculator is for Ocala and Marion County owners who want to ask a builder for the right number of wash bays, a floor that falls to the drain, and a wash water plan. It builds on our wash rack design and drainage guide and the stall size and layout guide.
En español: calculadora de lavadero de caballos y drenaje
| Your numbers | Result |
|---|
Example: 8 horses on a wash day, 5 in the busiest hour, average horses, 1 inch of fall per 6 ft, 12 minutes at 8 gpm, 5 wash days a week.
How the numbers are worked out
- Bays.
Horses in the busiest hour divided by 2 baths per bay per hour, rounded up. 5 horses: 5 / 2 = 2.5, so 3 bays. The 2 per hour is our planning assumption.
- Bay size.
From the horse size: an average horse uses 10 x 12 ft (published roomier bay) up to 12 x 12 ft. Floor area is bays x width x length.
- Fall.
Length in inches x slope. 12 ft = 144 in x 1.39% = 2 in over the length. Over a 10 ft width it is about 1.7 in.
- Drains.
One drain point per bay, so 3. Each needs a hair trap and a cleanout.
- Water.
Flow x minutes = water per bath: 8 gpm x 12 min = 96 gal. x 8 horses = 768 gal a wash day. x 5 days = 3,840 gal a week. The range is minus and plus 20%.
Slope: how much fall over a 12 ft bay
| Floor slope | Feet per 1 in of fall | Fall over 12 ft (in) |
|---|---|---|
| 1 | 8.3 | 1.4 |
| 1.4 | 6 | 2 |
| 2 | 4.2 | 2.9 |
The default is the commonly cited 1 inch per 6 feet. Pick the slope your finisher can form; the horse still has to stand on it.
Wash water by horses per wash day
| Horses per wash day | Per wash day (gal) | Per week, 5 days (gal) |
|---|---|---|
| 4 | 384 | 1,920 |
| 8 | 768 | 3,840 |
| 12 | 1,152 | 5,760 |
| 20 | 1,920 | 9,600 |
At 8 gpm for 12 minutes a horse (96 gal). Our planning assumption for flow and minutes; use your own numbers in the calculator.
Bay size by horse size
| Horse size | Low end (ft) | High end (ft) | Basis |
|---|---|---|---|
| Pony or small horse | 8 x 12 | 10 x 12 | Published minimum and roomier size |
| Average horse | 10 x 12 | 12 x 12 | Published roomier size; 12 x 12 is our assumption |
| Large horse or draft | 12 x 12 | 12 x 14 | Our planning assumption |
What the sources say
| Source | Figure | What it says |
|---|---|---|
| Equine facility guides | 8 x 12 / 10 x 12 ft | Common minimum wash rack and a roomier single bay. Typical figures, not code. |
| Commonly cited design figure | 1 in per 6 ft | Gentle fall to the drain, about 1.4%. See our wash rack guide. |
| Penn State Extension | Drain at side or back | Horses can balk at drain covers; a grassed filter area can treat wash stall wastewater. |
| FDACS Equine BMP, 2024 | 50 ft | Wash areas at least 50 ft from surface waters and domestic septic drainfields; direct runoff to a well-vegetated area. |
| FDEP small-scale horse guide, 2013 | 50 ft | Wash areas at least 50 ft from waterbodies, wells and septic drainfields; runoff to a swale or rain garden. |
| New York AEM worksheet | Less water, less to treat | Reducing wash water cuts the volume that needs treatment. A New York document, shown as general guidance. |
| Our assumptions | 2 baths per bay per hour; 1 drain per bay; 4, 8 and 12 gpm | Planning figures. Not published rules. |
Measure before you build. Fill a 5-gallon bucket from the hose you will use, and time one real bath. Two real numbers beat any typical figure.
Notes for an Ocala barn
- Where the water goes. Wash water carries sand, hair, manure and soap. Decide the discharge first; see the wash rack drainage guide.
- Soil. Sandy soil drains fast and filters little. See site soils in Marion County.
- Manure. Pick up manure in the wash area before rinsing. See manure management.
- Water supply. Check that your pump and line can supply the flow you chose. See water and power for a Marion County barn.
- Roof water. A covered rack adds roof runoff, which should not enter the wash drain. See the gutter and rainwater calculator.
What this calculator does not cover
- Price. No prices here. See horse barn cost in Ocala.
- Filter strip size. Area depends on soil, slope and the approved design. This tool only gives the volume to design for.
- Septic or sewer. Do not assume the drain can tie into a septic system. Ask the Florida Department of Health in Marion County.
- Permits and code. Plumbing and drainage rules are for the county. See horse barn permits in Marion County.
Questions to ask a builder about the wash rack
- How many bays, and what is each bay size, clear of the wall and ties? Give the builder the range from this page.
- What slope is the floor formed to, and in which direction?
- How many drains, where, and with what hair trap and cleanout?
- Where does the water go, and who approved it?
- Is the wash rack on the permit plan? Use the quote comparison worksheet.
Next step: get a written quote
This tool estimates space and water, not the price or the job. We are a referral service, not a builder; we connect visitors with independent local providers. Use the form below to ask independent local builders for a written quote, and read how to choose a horse barn builder first. To compare bids side by side, use the bid comparison calculator.
Sources
- Florida Equine Operations: Water Quality and Water Quantity BMPs, 2024 Edition, FDACS: Equine Washing Areas, 50 ft setback, runoff to a well-vegetated area.
- Water Quality Protection for Small-Scale Horse Operations, Florida DEP, 2013: Managing Horse Washing.
- Horse Stall Design and Horse Stable Flooring and Drainage, Penn State Extension: drain at side or back, textured floor, grassed filter for wash water.
- AEM Tier 2 Worksheet: Horse Wash Rack/Stall, New York State Agriculture and Markets: use less water, fewer gallons to treat.
- Iowa State University Extension: how to time a bucket fill to find your flow in gallons per minute.
- Wash rack planning, Cherry Hill, horsekeeping.com: 8 x 12 ft minimum. Size and slope ranges are typical figures, not code.
Frequently asked questions
How big should a horse wash rack be?
Equine facility guides commonly give 8 by 12 feet as the minimum and 10 by 12 feet for a roomier single bay. This calculator starts there and adds room for big horses and drafts as our planning assumption.
How much slope does a horse wash rack floor need?
A commonly cited figure is about 1 inch of fall per 6 feet, which is about 1.4 percent. Over a 12 foot bay that is a 2 inch fall. The calculator also shows 1 and 2 percent so you can see the difference.
How many drains does a horse wash rack need?
One drain point per bay is our planning assumption. Penn State advises putting the drain at the side or back, away from where the horse stands, and a trench along the back wall is common. Your plumber and builder set the real layout.
How much water does a horse bath use?
No extension guide we know of publishes one number, because it depends on hose flow and how long the water runs. This calculator multiplies your hose flow by your minutes. Time a 5-gallon bucket to measure your flow.
How far must a wash rack be from a well or septic field?
Florida's equine best management practices say to put wash areas at least 50 feet from surface waters and domestic septic drainfields and to direct runoff to a well-vegetated area. FDEP's small-scale horse guide also lists wells in that 50 foot setback. Marion County or your well and septic rules may ask for more.