A proper irrigation schedule will reduce water losses. However, due to their high costs, water optimization tools have so far been available to large farms only. This low-cost, portable water irrigation planner requires the selection of a crop, its maturity stage, choice of a location in California and sprinkler type to compute the optimal daily irrigation time. This is useful for small farms, gardeners and hobby farmers who wish to lower their consumption of water while maximizing crop growth. The smart phone app uses the soil water balance approach along with soil and weather data provided via application programming interfaces (API). As dry season and water deficiencies are increasingly frequent, the app will enhance the effectiveness of irrigation by computing the average daily irrigation duration in the current month for a range of crops in California. The app relies on locally-calibrated coefficients for a range of crops, thereby minimizing user input. Designed to optimize irrigation cycles for a wide range of crops in California while remaining user-friendly, this smartphone app was modelled after the soil water balance approach to computer crop water requirements. Water requirements for plants depend on water inputs, including rainfall and irrigation, and water outputs, that is, losses due to runoff, deep percolation and evapotranspiration (based on the penman monteith equation). Net irrigation requirement is the quantity of water to apply for crop growth. Gross irrigation is the quantity of water to be applied taking into consideration crop water requirement and water losses caused by inefficiencies in the irrigation system. The app computes effective rainfall using the USDA-SCS (1970) formula, calculates the soil water storage factor (the available water in storage that can be used by crops between irrigation events) and crop evapotranspiration to determine the average daily irrigation runtime. Reference evapotranspiration is that from a grass surface that is well watered. Crop evapotranspiration is defined as the sum of evaporation from the land surface plus transpiration from plant leaves. Water from soil in the vicinity of the plant and water from the stems and leaves of the plant evaporates into the air. The plant roots draw water from the soil up into the leaves, but some of it is lost due to the plant transpiration that varies according to weather conditions such as temperature, humidity, sunlight, precipitation, soil type and wind. The app provides the optimal irrigation cycle (number of minutes of irrigation per day for a specific crop) in the current month for any selected location in California. This advanced mobile phone application requires minimal technical skills and inputs from users (e.g. crop type, maturity stage, sprinkler precipitation rate). All other data necessary to compute the optimal irrigation cycle including average monthly rainfall, evapotranspiration, root zone depth, maximum allowable depletion (the fraction/percentage of plant available water that is to be depleted from the active root zone before irrigation is applied) and available water content (the amount of water actually available to the plant for its growth) and soil data are retrieved or computed by the app from existing soil maps such as the Harmonized World Soil Database and weather stations in California.