diff --git a/Sources/ApiController/Extensions/RoomPressure.swift b/Sources/ApiController/Extensions/RoomPressure.swift
index 136296c..3e67a2d 100644
--- a/Sources/ApiController/Extensions/RoomPressure.swift
+++ b/Sources/ApiController/Extensions/RoomPressure.swift
@@ -38,7 +38,116 @@ public extension RoomPressure.Request {
   // ])
 
   func respond(logger: Logger) async throws -> RoomPressure.Response {
-    fatalError()
+    switch self {
+    case let .knownAirflow(request):
+      return try await calculateKnownAirflow(request, logger)
+    case let .measuredPressure(request):
+      return try await calculateMeasuredPressure(request, logger)
+    }
+  }
+
+  private func calculateKnownAirflow(
+    _ request: RoomPressure.Request.KnownAirflow,
+    _ logger: Logger
+  ) async throws -> RoomPressure.Response {
+    try request.validate()
+
+    let totalLeakageArea = calculateDoorLeakageArea(
+      doorWidth: request.doorWidth,
+      doorHeight: request.doorHeight,
+      doorUndercut: request.doorUndercut
+    )
+
+    // Net free area (in^2)
+    let netFreeArea = (request.supplyAirflow / Self.targetVelocity) * 144.0
+    let grilleDimensions = try getStandardGrilleSize(
+      requiredArea: netFreeArea,
+      selectedHeight: request.preferredGrilleHeight.rawValue
+    )
+    let (standardDuctSize, actualVelocity) = calculateDuctMetrics(for: request.supplyAirflow)
+
+    return .init(
+      grilleSize: .init(width: grilleDimensions.width, height: grilleDimensions.height, area: netFreeArea / 144),
+      ductSize: .init(diameter: standardDuctSize, velocity: actualVelocity),
+      warnings: generateWarnings(
+        roomPressure: request.targetRoomPressure,
+        actualVelocity: actualVelocity,
+        totalLeakageArea: totalLeakageArea,
+        netFreeArea: netFreeArea
+      )
+    )
+  }
+
+  private func calculateMeasuredPressure(
+    _ request: RoomPressure.Request.MeasuredPressure,
+    _ logger: Logger
+  ) async throws -> RoomPressure.Response {
+    try request.validate()
+
+    let totalLeakageArea = calculateDoorLeakageArea(
+      doorWidth: request.doorWidth,
+      doorHeight: request.doorHeight,
+      doorUndercut: request.doorUndercut
+    )
+
+    let pressureInchesWaterColumn = request.measuredRoomPressure * Self.pascalToIWCMultiplier
+    let calculatedAirflow = totalLeakageArea * 4005 * sqrt(pressureInchesWaterColumn)
+    // in^2
+    let netFreeArea = (calculatedAirflow / Self.targetVelocity) * 144
+    let grilleDimensions = try getStandardGrilleSize(
+      requiredArea: netFreeArea,
+      selectedHeight: request.preferredGrilleHeight.rawValue
+    )
+    let (standardDuctSize, actualVelocity) = calculateDuctMetrics(for: calculatedAirflow)
+
+    return .init(
+      grilleSize: .init(width: grilleDimensions.width, height: grilleDimensions.height, area: netFreeArea / 144),
+      ductSize: .init(diameter: standardDuctSize, velocity: actualVelocity),
+      calculatedAirflow: calculatedAirflow,
+      warnings: generateWarnings(
+        roomPressure: request.measuredRoomPressure,
+        actualVelocity: actualVelocity,
+        totalLeakageArea: totalLeakageArea,
+        netFreeArea: netFreeArea
+      )
+    )
+  }
+
+  private func calculateDuctMetrics(for airflow: Double) -> (diameter: Int, velocity: Double) {
+    let ductArea = airflow / Self.targetVelocity
+    let ductDiameter = sqrt((4 * ductArea) / Double.pi) * 12
+    let standardDuctSize = getStandardDuctSize(for: Int(ductDiameter))
+    let actualVelocity = airflow / (Double.pi * pow(Double(standardDuctSize) / 24, 2))
+
+    return (standardDuctSize, actualVelocity)
+  }
+
+  private func generateWarnings(
+    roomPressure: Double,
+    actualVelocity: Double,
+    totalLeakageArea: Double,
+    netFreeArea: Double
+  ) -> [String] {
+    var warnings = [String]()
+
+    if roomPressure > 3 {
+      warnings.append(
+        "Room pressure exceeds 3 Pascals - consider reducing pressure differntial."
+      )
+    }
+
+    if totalLeakageArea > netFreeArea / 144 * 0.5 {
+      warnings.append(
+        "Door leakage area is significant - consider reducing gaps or increasing grille size."
+      )
+    }
+
+    if actualVelocity > Self.maxVelocity {
+      warnings.append(
+        "Return air velocity exceeds maximum recommended velocity - consider next size up duct."
+      )
+    }
+    return []
   }
 
   private func getStandardDuctSize(for calculatedSize: Int) -> Int {
@@ -74,3 +183,40 @@ public extension RoomPressure.Request {
     case invalidPreferredHeight
   }
 }
+
+extension RoomPressure.Request.KnownAirflow {
+  func validate() throws {
+    guard targetRoomPressure > 0 else {
+      throw ValidationError(message: "Target room pressure should be greater than 0.")
+    }
+    guard doorWidth > 0 else {
+      throw ValidationError(message: "Door width should be greater than 0.")
+    }
+    guard doorHeight > 0 else {
+      throw ValidationError(message: "Door height should be greater than 0.")
+    }
+    guard doorUndercut > 0 else {
+      throw ValidationError(message: "Door undercut should be greater than 0.")
+    }
+    guard supplyAirflow > 0 else {
+      throw ValidationError(message: "Supply airflow should be greater than 0.")
+    }
+  }
+}
+
+extension RoomPressure.Request.MeasuredPressure {
+  func validate() throws {
+    guard measuredRoomPressure > 0 else {
+      throw ValidationError(message: "MeasuredPressure room pressure should be greater than 0.")
+    }
+    guard doorWidth > 0 else {
+      throw ValidationError(message: "Door width should be greater than 0.")
+    }
+    guard doorHeight > 0 else {
+      throw ValidationError(message: "Door height should be greater than 0.")
+    }
+    guard doorUndercut > 0 else {
+      throw ValidationError(message: "Door undercut should be greater than 0.")
+    }
+  }
+}
diff --git a/Sources/ApiController/ValidationError.swift b/Sources/ApiController/ValidationError.swift
new file mode 100644
index 0000000..7a2bc77
--- /dev/null
+++ b/Sources/ApiController/ValidationError.swift
@@ -0,0 +1,3 @@
+struct ValidationError: Error {
+  let message: String
+}
diff --git a/Sources/Routes/Models/RoomPressure.swift b/Sources/Routes/Models/RoomPressure.swift
index 9788832..6d7fee2 100644
--- a/Sources/Routes/Models/RoomPressure.swift
+++ b/Sources/Routes/Models/RoomPressure.swift
@@ -87,10 +87,10 @@ public enum RoomPressure {
 
     public struct DuctSize: Codable, Equatable, Sendable {
 
-      public let diameter: Double
+      public let diameter: Int
       public let velocity: Double
 
-      public init(diameter: Double, velocity: Double) {
+      public init(diameter: Int, velocity: Double) {
         self.diameter = diameter
         self.velocity = velocity
       }
@@ -99,11 +99,11 @@ public enum RoomPressure {
 
     public struct GrilleSize: Codable, Equatable, Sendable {
 
-      public let width: Double
-      public let height: Double
+      public let width: Int
+      public let height: Int
       public let area: Double
 
-      public init(width: Double, height: Double, area: Double) {
+      public init(width: Int, height: Int, area: Double) {
         self.width = width
         self.height = height
         self.area = area