--- title: "Dewatering Pump Flow Calculation: How to Select Required Flow Rate" date: 2026-08-17T06:37:24Z modified: 2026-08-31T06:56:45Z permalink: "https://www.defucc.com/blog/buying-guide/pump-flow-calculation-dewatering-pump/" type: post status: publish excerpt: "" wpid: 2857 categories: - buying guide featured_image: "https://www.defucc.com/wp-content/uploads/2026/08/image-9.webp" featured_image_alt: Dewatering pump flow calculation timestamp: 2026-08-31T06:56:45Z tags: - buying guide --- ## **Quick Answer:** Required pump flow rate equals the volume of water to remove divided by the available pumping time. If water continues entering the site, add that inflow to the result. The formula is required flow = (water volume ÷ pumping time) + continuous Inflow. For example, 6,000 m³ in 24 hours requires 250 m³/h before inflow. ## **What Is Pump Flow Rate?** Pump flow reveals the volume of water that a pump has moved over a given period. It’s usually expressed in gallons per minute (GPM), but some also use liters per second (L/s) or cubic meters per hour (m³/h) to denote it. When it comes to dewatering, the required flow depends on the amount of water and how quickly you need to remove it. A construction sump may require a few cubic meters per hour, whereas a flooded mine may need hundreds. So instead of beginning with the maximum capacity listed or a pump, it’s better to work from the conditions on your site. ## **What Is the Basic Pump Flow Calculation?** If you have a known volume of standing water and little or no water is entering the site, the pump flow calculation is: **Required Flow = Water Volume ÷ Pumping Time** Assuming that a pit has 6,000 m³ of water and you need to drain it within 24 hours: **6,000 ÷ 24 = 250 m³/h** You need a flow rate of 250 m³/h. A pump GPM calculation works the same way. If 30,000 gallons must be removed within 100 minutes: **30,000 ÷ 100 = 300 GPM** You can use this calculation for sumps, tanks, and other jobs where the water volume and the time available to remove it are known to you. ## **What If Water Keeps Entering the Site?** On many dewatering jobs, pumping doesn’t stop new water from arriving. Groundwater may seep into an excavation, rainwater may collect in a mine pit, or process water may keep flowing into a sump. Add that ongoing inflow to the amount you already need to pump: **Required Flow = (Stored Water ÷ Drawdown Time) + Continuous Inflow** Suppose a mine pit holds 15,000 m³ of water. It needs to be pumped down within 48 hours, while groundwater continues entering at 120 m³/h. First, calculate the flow needed for the stored water: **15,000 ÷ 48 = 312.5 m³/h** Then add the inflow: **312.5 + 120 = 432.5 m³/h** The required pump flow rate is therefore 432.5 m³/h under those conditions. Leave a suitable reserve capacity if rainfall, seepage, or process water could push the actual inflow above your estimate. ## **How Can You Measure Water Inflow?** If you’re unaware of the inflow rate, see how quickly the water level rises when pumping halts. If it’s a sump or pit with a reasonably uniform area, measure the water depth increase over a set time period. You can find out how much water entered by using the change in depth and surface area, and then dividing that volume by the time elapsed: **Inflow Rate = Increase in Water Volume ÷ Time** For a small stream of incoming water, you can collect it in a container of known volume and time how long it takes to fill. Try to measure under conditions that represent the job. An excavation after heavy rain may receive far more water than it does on a dry day. ## **How Does Flow Demand Differ by Application?** The same [pump capacity calculation](https://www.defucc.com/wp-content/uploads/wp-mfa-exports/page/pump-head-calculator.md) can be used across different jobs, but the sources of incoming water aren’t always the same. In mine dewatering, you may have stored water plus groundwater, rainfall, and process water to handle. In one [DEFU mine dewatering ](https://www.defucc.com/wp-content/uploads/wp-mfa-exports/page/mine-dewatering-pump-manufacturer.md)project in Ghana, the required system capacity reached 1,000 m³/h. For [construction dewatering](https://www.defucc.com/wp-content/uploads/wp-mfa-exports/post/dewatering-pumps-in-construction.md), groundwater seepage and surface runoff can keep adding water to foundation pits, excavations, and sumps. Your required flow has to account for that incoming water as well as any volume you need to remove within a deadline. With [agricultural irrigation](https://www.defucc.com/wp-content/uploads/wp-mfa-exports/page/agriculture-pump.md), the starting point is the amount of water the field or crop needs within the available operating time. Irrigated area, required water depth, operating hours, and application efficiency can all affect that requirement. ## **How Do You Convert Pump Flow Units?** If your site figures and pump specifications use different units, these conversions can help: **·** **1 m³/h ≈ 4.40 GPM** **·** **1 L/s ≈ 15.85 GPM** **·** **1 L/min ≈ 0.264 GPM** **· 1 L/s = 3.6 m³/h** Keep the same units throughout each calculation. For example, if your water volume is in gallons and your required result is GPM, use pumping time in minutes. Looking for a Professional Mine Dewatering Solution? Explore our advanced facilities and cutting-edge processes that ensure precision, quality, and innovation in every pump. [Discover Our Mine Dewatering Case](https://www.defucc.com/wp-content/uploads/wp-mfa-exports/page/mine-dewatering-pump-manufacturer.md) ## **What Do You Do With the Required Flow Rate?** Once you know the required flow, record the normal and expected peak inflow along with the time available to remove the water. This gives you a clear flow requirement to send to the manufacturer. You can [submit your required flow to DEFU](https://defupump.com/add/contact/) for a pump recommendation based on the application, whether you’re dewatering a mine pit or a construction site or supplying water for agricultural irrigation. [Get Free Quote](https://www.defucc.com/wp-content/uploads/wp-mfa-exports/page/contact.md) Welcome to Our Blog! Professional Dewatering Pump Manufacturer Written by the Engineering & Solutions Team at WENZHOU DEFU MACHINERY CO., LTD. Backed by 3 manufacturing bases and over 10 years of expertise as a leading dewatering pump manufacturer, our team is dedicated to sharing practical dewatering pump insights and engineered fluid handling solutions for global projects. [ Contact Us → ](https://www.defucc.com/wp-content/uploads/wp-mfa-exports/page/contact.md) ![WENZHOU DEFU MACHINERY CO., LTD. - Dewatering Pump Manufacturer](https://defucc.b-cdn.net/wp-content/uploads/2025/06/2-1-1--768x534.jpg)