diff --git a/src/humidity/absolute.clj b/src/humidity/absolute.clj
index 38a93a8..d686e7c 100644
--- a/src/humidity/absolute.clj
+++ b/src/humidity/absolute.clj
@@ -31,8 +31,7 @@
 
 ;; AH=(RH × Ps)​​/(Rw ​× T × 100)
 
-(defn abs-humidity [& {:keys [relative-humidity saturation-pressure temperature-kelvin]}]
-  (when (and relative-humidity saturation-pressure temperature-kelvin)
+(defn abs-humidity [& {:keys [relative-humidity saturation-vapour-pressure temperature-kelvin]}]
+  (when (and relative-humidity saturation-vapour-pressure temperature-kelvin)
     ;; I don't know why I'm having to multiply by a thousand here...
-    (* (/ (* relative-humidity saturation-pressure) Rw temperature-kelvin) 1000))
-  )
+    (* (/ (* relative-humidity saturation-vapour-pressure) Rw temperature-kelvin) 1000)))
diff --git a/src/humidity/actual_vp.clj b/src/humidity/actual_vp.clj
new file mode 100644
index 0000000..3e742f5
--- /dev/null
+++ b/src/humidity/actual_vp.clj
@@ -0,0 +1 @@
+(ns humidity.actual-vp)
diff --git a/src/humidity/core.clj b/src/humidity/core.clj
index 16f5054..6267107 100644
--- a/src/humidity/core.clj
+++ b/src/humidity/core.clj
@@ -1,6 +1,7 @@
 (ns humidity.core
   (:require [humidity.absolute :refer [abs-humidity]]
             [humidity.constants :refer [celsius-offset Pa]]
+            [humidity.relative :refer [rel-humidity]]
             [humidity.saturation-vp :as saturation-vp]
             [humidity.utils :refer [?assoc]]
             [humidity.wet-bulb :refer [wet-bulb-temperature]]))
@@ -28,6 +29,7 @@
                                       (if pressure-millibars (* pressure-millibars 100) Pa))
               :pressure-millibars (when-not pressure-millibars
                                     (if pressure-pascals (/ pressure-pascals 100) (/ Pa 100)))
+              :relative-humidity (when-not relative-humidity (rel-humidity packet))
               :saturation-vapour-pressure (saturation-vp/saturation-vp packet))))
 
 
diff --git a/src/humidity/relative.clj b/src/humidity/relative.clj
index 247f7e3..23e1b72 100644
--- a/src/humidity/relative.clj
+++ b/src/humidity/relative.clj
@@ -1,7 +1,7 @@
 (ns ^{:authority "https://journals.ametsoc.org/view/journals/apme/35/4/1520-0450_1996_035_0601_imfaos_2_0_co_2.xml/"
       :author "simon"
       :doc "Function to return relative humidity."} humidity.relative
-  (:require [humidity.constants :refer [celsius-offset e]]
+  (:require [humidity.constants :refer [celsius-offset e Rw]]
             [humidity.utils :refer [expt]]))
 
 (def ^:const magic-numbers
@@ -13,22 +13,43 @@
 
 (def C magic-numbers)
 
+;;        / / relative-humidity x saturation-pressure \ \
+;;        | | --------------------------------------- | |
+;; 1000 * | \                  Rw                     / | = absolute-humidity
+;;        | ------------------------------------------- |
+;;        \             temperature-kelvin              /
+;;
+;; divide both sides by 1000 ==>
+;; / / relative-humidity x saturation-pressure \ \
+;; | | --------------------------------------- | |   / absolute-humidity \
+;; | \                  Rw                     / | = | ----------------- |
+;; | ------------------------------------------- |   \       1000        /
+;; \             temperature-kelvin              /
+;;
+;; multiply both sides by temperature-kelvin ==>
+;; / relative-humidity x saturation-pressure \   / absolute-humidity x temperature-kelvin \
+;; | --------------------------------------- | = | -------------------------------------- |
+;; \                  Rw                     /   \                  1000                  /
+;;
+;; Multiply both sides by Rw ==>
+;;                                           absolute-humidity x temperature-kelvin x Rw
+;; relative-humidity x saturation-pressure = -------------------------------------------
+;;                                                                 1000
+;;
+;; Divide both sides by saturation-pressure ==>
+;;
+;;                      absolute-humidity x temperature-kelvin x Rw
+;; relative-humidity  = -------------------------------------------
+;;                               1000 x saturation-pressure
+
 (defn rel-humidity
   "Returns relative humidity (as a percentage, 0...100) given values for dew
-   point and temperature in either Celsius or Kelvin."
-  [& {:keys [dew-point temperature-celsius temperature-kelvin]}]
-  (cond (and dew-point (or temperature-celsius temperature-kelvin))
-    (let [Tc (if temperature-celsius
-               temperature-celsius
-               (- temperature-kelvin celsius-offset))]
-      (* 100 (/ (expt e (/ (* (C 1) dew-point)
-                           (+ (C 2) dew-point)))
-                (expt e (/ (* (C 1) Tc)
-                           (+ (C 2) Tc))))))))
-
-;; Absolute Humidity (grams/m3) = 6.112 × e^[(17.67 × T)/(T+243.5)] × rh × 2.1674
-;;                                -----------------------------------------------
-;;                                                   (273.15+T)
-;;
-;; => /
-;;
+   point and temperature in Celsius or for absolute humidity, saturation vapour pressure temperature Kelvin."
+  [& {:keys [absolute-humidity dew-point saturation-vapour-pressure temperature-ke temperature-kelvin] :as packet}]
+    (cond (and dew-point temperature-ke)
+          (* 100 (/ (expt e (/ (* (C 1) dew-point)
+                               (+ (C 2) dew-point)))
+                    (expt e (/ (* (C 1) temperature-ke)
+                               (+ (C 2) temperature-ke)))))
+          (and absolute-humidity saturation-vapour-pressure temperature-kelvin)
+          (/ (* absolute-humidity temperature-kelvin Rw) (* 1000 saturation-vapour-pressure))))
diff --git a/test/humidity/absolute_test.clj b/test/humidity/absolute_test.clj
index b983e1f..9a13e6f 100644
--- a/test/humidity/absolute_test.clj
+++ b/test/humidity/absolute_test.clj
@@ -8,6 +8,6 @@
     (let [min-expected 39.58
           max-expected 39.59
           actual (abs-humidity :relative-humidity 100
-                               :saturation-pressure 56.29
+                               :saturation-vapour-pressure 56.29
                                :temperature-kelvin (+ 35 celsius-offset))]
       (is (< min-expected actual max-expected)))))
diff --git a/test/humidity/relative_test.clj b/test/humidity/relative_test.clj
index 3a9d52b..9d7c5fd 100644
--- a/test/humidity/relative_test.clj
+++ b/test/humidity/relative_test.clj
@@ -1,9 +1,14 @@
 (ns humidity.relative-test
   (:require [clojure.test :refer [deftest is testing]]
             [humidity.constants :refer [celsius-offset]]
-            [humidity.relative :refer [rel-humidity]]))
+            [humidity.relative :refer [rel-humidity rel-humidity-2]]))
 
 (deftest relative-humidity-test
+  (testing "definitional"
+    (let [min-expected 99
+      max-expected 101
+      actual (rel-humidity-2 :absolute-humidity 39.58 :saturation-vapour-pressure 5622.49 :temperature-kelvin (+ 35 celsius-offset))]
+    (is (< min-expected actual max-expected))))
   (testing "Online examples"
     (let [min-expected 99
       max-expected 101