diff --git a/PurpleMenu/AppDelegate.swift b/PurpleMenu/AppDelegate.swift
index b9e42eb..cad2180 100644
--- a/PurpleMenu/AppDelegate.swift
+++ b/PurpleMenu/AppDelegate.swift
@@ -94,9 +94,20 @@ class AppDelegate: NSObject, NSApplicationDelegate {
                   let humidity = Float(humidityStr)
             else { return }
 
-            debugPrint("id = \(self.sensorViewModel.sensorId), pm25 = \(pm25), pm25Cf1 = \(pm25Cf1), RH = \(humidity)")
-
-            let aqi = self.pmToEPA(paCf1: (pm25Cf1 + pm25Cf1B) * 0.5, humidity: humidity)
+            debugPrint("id = \(self.sensorViewModel.sensorId), conversion = \(UserDefaults.standard.conversion), pm25 = \(pm25), pm25Cf1 = \(pm25Cf1), RH = \(humidity)")
+
+            let aqi: Int
+
+            switch Conversion(rawValue: UserDefaults.standard.conversion) ?? Conversion.epa {
+            case .none:
+                aqi = self.pmToAQI(pm25)
+            case .epa:
+                aqi = self.pmToEPA(paCf1: (pm25Cf1 + pm25Cf1B) * 0.5, humidity: humidity)
+            case .aqandu:
+                aqi = self.pmToAQandU(pm: pm25)
+            case .lrapa:
+                aqi = self.pmToLRAPA(paCf1: pm25Cf1)
+            }
 
             DispatchQueue.main.async {
                 self.statusBarItem.button?.title = "aqi=\(aqi)"
diff --git a/PurpleMenu/ContentView.swift b/PurpleMenu/ContentView.swift
index 76aa7fb..9a20a63 100644
--- a/PurpleMenu/ContentView.swift
+++ b/PurpleMenu/ContentView.swift
@@ -23,6 +23,13 @@ struct ContentView: View {
                     .frame(width: 80)
                 }
                 .frame(width: g.size.width / 2, alignment: /*@START_MENU_TOKEN@*/.center/*@END_MENU_TOKEN@*/)
+                Picker("Conversion", selection: $sensor.conversion) {
+                    ForEach(Conversion.allCases) { conversion in
+                        Text(conversion.rawValue)
+                    }
+                }
+                .frame(width: 180)
+                .pickerStyle(PopUpButtonPickerStyle())
                 HStack {
                     Button("Update") {
                         sensor.update()
diff --git a/PurpleMenu/Sensor.swift b/PurpleMenu/Sensor.swift
index 7e3e05a..83ba6cf 100644
--- a/PurpleMenu/Sensor.swift
+++ b/PurpleMenu/Sensor.swift
@@ -7,6 +7,15 @@
 
 import Foundation
 
+enum Conversion: String, CaseIterable, Identifiable {
+    case none = "None", epa = "US EPA", aqandu = "AQandU", lrapa = "LRAPA"
+
+    var id: String { self.rawValue }
+    var description: String {
+        return self.rawValue
+    }
+}
+
 struct Sensor: Codable {
     let mapVersion : String?
     let baseVersion : String?
diff --git a/PurpleMenu/SensorViewModel.swift b/PurpleMenu/SensorViewModel.swift
index 159286d..236b7ca 100644
--- a/PurpleMenu/SensorViewModel.swift
+++ b/PurpleMenu/SensorViewModel.swift
@@ -9,13 +9,17 @@ import Foundation
 import Combine
 
 final class SensorViewModel: ObservableObject {
-    static let userDefaultsKey = "SensorId"
-
     @Published var sensorId = UserDefaults.standard.sensorId
+    @Published var conversion = UserDefaults.standard.conversion {
+        didSet {
+            self.update()
+        }
+    }
 
     func update() {
-        if !sensorId.isEmpty {
-            UserDefaults.standard.sensorId = sensorId
-        }
+        guard !sensorId.isEmpty else { return }
+
+        UserDefaults.standard.sensorId = sensorId
+        UserDefaults.standard.conversion = conversion
     }
 }
diff --git a/PurpleMenu/UserDefaultsExtension.swift b/PurpleMenu/UserDefaultsExtension.swift
index 79b7d9c..d0729be 100644
--- a/PurpleMenu/UserDefaultsExtension.swift
+++ b/PurpleMenu/UserDefaultsExtension.swift
@@ -16,4 +16,13 @@ extension UserDefaults {
             set(newValue, forKey: "sensor_id")
         }
     }
+
+    @objc var conversion: String {
+        get {
+            return string(forKey: "conversion") ?? Conversion.epa.rawValue
+        }
+        set {
+            set(newValue, forKey: "conversion")
+        }
+    }
 }
diff --git a/README.md b/README.md
index d8e166b..ff19ff4 100644
--- a/README.md
+++ b/README.md
@@ -1,9 +1,6 @@
 # PurpleMenu
-I wrote this to display the nearest Purple Air AQI with US EPA conversion in my macOS menu bar, tested in Catalina and Big Sur.
+I wrote this to display my nearest Purple Air AQI with US EPA conversion in my macOS menu bar, tested in Catalina and Big Sur.
 
-For now you can change the `sensorId` [here](https://github.com/ayn/PurpleMenu/blob/master/PurpleMenu/AppDelegate.swift#L67) to monitor your preferred Purple Air sensor.
+There's a basic UI to change sensor to any public Purple Air outdoor sensor, and choose between US EPA, AQandU, LRAPA, and no conversion.
 
-## TODOS
-
-* Settings UI
-  * choose which conversion (or none) to use
+To get a sensor ID, go to [Purple Air Map](http://www.purpleair.com/map?mylocation) in your browser, click/tap on a sensor on the map, look at the url in your browser's location bar, and find the `select=` parameter.